Is there a way to search the MUL for all plasma rifle equipped mechs?
Not exactly, but you can find some of them by searching for the AS heat trait, which I believe is HT. That will find any Plasma Rifle, Cannon, or bundle of flamer death.
You can also create a log in at this fan developed site and search for Plasma Rifles here:
http://mech.thompfam.net/The reason I don't mention it first is it won't sort cannon designs from customs. So if you can't spot the customs, refer to the MUL to weed them out.