As I understand the situation, a "spike and slab" prior in PROC MCMC cannot now be specified because this distribution requires a combination of a DGENERAL and GENERAL distribution. [Regarding "spike and slab", see the reference: https://www.mv.helsinki.fi/home/mjxpirin/HDS_course/material/HDS8_variable_selection.html]
The "spike and slab" prior for a predictor X would support "variable selection" when using PROC MCMC.
To fully implement the approach given in the reference above, the functionality of the R package called BoomSpikeSlab would be needed. See https://cran.r-project.org/web/packages/BoomSpikeSlab/index.html