On my CLS1840 head, the top clear filter is a glass IR filter, then the 3 dicro filters, and on the bottom a clear glass UV filter.

I think I was able to access and remove the UV filter by first removing the lamp assembly and IR filter, then zeroing the dicro filters, and finally removing 2 thin spring-steel clips, each clip held by 2 screws. Once you start dis-assembly, it will be easier than you first thought.

Underneath the glass UV filter, my CLS1840 has a glass diffuser. Once you get that far, you might as well clean the diffuser, too.

Like you mentioned, I can't imagine the UV filter being glass, as it gets pretty hot.