Okay, I seem to have made a little headway. Using two table files with switching seems to work with romjuice.
!5C
B1=ァ
Is in my kata.tbl
I put !5C in both of the tables and told romjuice to use hira.tbl and additionally use kata.tbl with -t. I still have a bunch of values unfinished (I tested with only one katakana for kicks) but
<$05>ァ<$B8><$BC><$AE><$DD><$3D><$B9><$DE><$B0><$D1>が<$3C><$CD><$C0>な<$3C>ひと<$00>こんにちは!
Is just what I expected.
Guess what was my first discovered relative string

Now I have to figure out how I could reinsert that switch. I know I might not
need the katakana space but I'd like to know how to use it, maybe investigate if upper and lower case is possible here using that tile space.