You can initiate a blogger-search by hard-coding the links into your sidebar too (cosmos search stylee) and then inserting the relevant keywords into all your post titles. What with this being Freshblog, and all, the results (surprise, surprise) will stand in for categories.
See Netcf2 for instructions, and for the sidebar list code.
Update 11/22: Daniel has written a code-generator that will build the sidebar menu for you. Cool!