I was wondering if there is any way to assign the sliders in the way you can assign the knobs?

No.

As mentioned - sliders are not available as control sources for modulating parameters. However, if you want to indirectly link a slider to use as a modulation source - you can do that.

This would involve "burning resources" to act as intermediary links between the sliders and parameters. When you assign a knob to something - you do so without consuming anything except a destination link. If you want indirectly link the sliders - you would need to, at the least, consume a PART (1-8) that can be initially triggered to produce a volume output.

So the high-level is that you would have a PART that, once you press any piano key, starts to produce output and will not change this output unless you move the slider to affect the amplitude of this output. The sliders can modulate the volume of PARTs - so this is the first step. To have the slider raise and lower the amplitude of the PART which is constantly on. I always use an FM part because they're easier to deal with for this purpose. A pure sine wave that does not change pitch (fixed ratio) and also I make the FM PART monophonic (no chords) to keep the amplitude constant no matter how many fingers are pushing keys at once. The release time I set to a value that means "hold forever" when you stop pressing piano keys. Also, set this PART's output to OFF so you do not hear it. The second step is to use an envelope follower. This converts the level of the "burn" PART to a value which can be a modulation source used just like assignable knobs and the rest.

Adjust the envelope follower hysteresis values to make the envelope follower either more tightly follow the slider or have a more "squishy" response.

Now that is really clever!

I will have to try that out. I want the slider to change the oscillator frequency of the ring modulator effect.
Currently I have a knob doing that, but it's not possible to manipulate the frequency in the way I want to.

FYI: the request for more direct assignment of the sliders has been suggested through ideascale (on Aug 22nd 2018).

https://yamahasynth.ideascale.com/a/dtd/More-Flexibility-In-Sliders-Faders/210040-45978

If you have anything to add, you may want to discuss those ideas there.

The idea covers not only the lack of flexibility (today) in assigning sliders to destinations - but also how polarity of the sliders cannot be changed. Which would be useful even if the sliders continued to only control volume. The organ drawbars are currently "push for louder" where the tonewheel organ drawbars are, for Hammond, "pull for louder". Also covered is better use of sliders for VSTi control without having to use a DAW Remote mode.

The idea currently states:

Currently, Yamaha sliders serve primarily a fixed function. Either they can be fixed to control various volumes (expression) or can be set to a DAW transport control. Ignoring DAW transport control - I would like the "normal" non-transport function of the sliders to be more flexible. Some examples:

1) Ability to reverse the polarity (individually) of each slider. Some uses for this would be for organ drawbar or to enable grabbing two sliders and slide both in the same direction to crossfade (without having to use another automation technique).

2) Ability to assign sliders as controller sources (which also allows for non-linear or non-monotonic behavior to the sliders - even for the same destination they would currently target without the change).

3) Ability to assign sliders (which, in some modes are only SysEx) to CC messages for VST control

... #2 would be the part which covers your initial question.

Yes, that is exactly right. I was initially surprised I couldn't assign the sliders like you can with the knobs.

I wasn't aware you could use them for DAW control either, but maybe this only applies to Montage and not MODX?

I keep getting close to the intended function with your "burn a Part" approach - the Ring Mod will change pitch
under control of Slider 4 through a customized Part 4 (FM with the 8 operators straight across) routed into the
Envelope Follower with no level to the output and that is set to modulate the Ring Mod pitch but it seems like
every time I am editing I have the same problem: I get what I want but many other things will change and I will
have to figure out how to get them back to where they were! This has been a problem for me ever since the
day I received my beloved MODX7 a few months ago.

DAW control isn't a part of MODX firmware as far as I know. Montage has this mode but not MODX. This may or may not change in the future with firmware updates. I'm completely in the dark on this front.