I would think that they would try and spawn ... anywhere they can. Mud, gravel, or sand banks with sparse vegetation is their "preferred" spawning habitat (we are talking Black Crappie here, right?) From your description, it sounds like they are, in fact, in the prespawn stage ... and looking for suitable bedding areas.
There's lakes I fish, here in KY, that have both White & Black Crappie ... but no vegetation growing. The Black Crappie spawn on/around the wood cover, that is in abundance in these lakes. So, yes ... they can & will use whatever cover or bottom composition is available, given good water quality/temps/conditions.
The Crappie will not "expel" the eggs, just to get rid of them. If conditions prevail, that prevents them from spawning, they'll keep their eggs and consume the nutrients for their own nourishment. But, instincts will tell them to try and spawn, even in marginal conditions. Even the survival of only a few of their offspring, is better than no survival of ALL of them.
If the grasses died out in 2004, and haven't returned (in 2yrs) ... has there been any attempt to transplant any aquatic vegetation to this lake ? Has there been any re-emergence of "any" type of vegetation ? If not ... sounds like you need to get busy and start planting some "condo's" (if it is legal) !!
....luck2ya ...... cp