All depends on how far along the spalting is. The very coolest looking ones are typically a little far gone for risers but can still be used in limbs.
You can still get good looking spalted that is solid though. The best risers have some sort of support anyway. In one piece bows, the glass on the back is usually enough. in three piece bows, risers backed with glass are much more durable especially with woods that could be questionable.
This glass can be reinforced woven like G10 or just clear glass. It reduces the flex of the riser tremendously. Phenolic will help also, but it flexes much more than glass.
An I beam in glass ...or phenolic will really stiffen it up, but not necessary if done properly and backed with glass.
Remember, if you put anything else inside a riser that is not wood, you will want to be very discriminating about how dry the pieces you use are.
Even though different species of woods expand and contract at different rates, shrinkage is less likely to cause separation at a glue line between woods than between any line next to a sandwitched piece of phenolic or glass. Phenolic and glass expand and contract very little.
BigJim