Good news! Although you can’t put clefs and key signatures anywhere you want on staves with 6 and 8 lines, it happens that the existing clefs can work to create an intavulatura:
The instruments with 6-line and 8-line staves were created using Library > Instruments. The F clef was modified to add a C clef above it using Library > Music Symbols. Both the C and F clefs were reduced to 65% of their normal size to keep them from overlapping on the 8-line staff. The 12-EDO tonality system was edited to add an accidental with two flats in one column with the pitch delta set to -1, and a custom key signature was created using this accidental and applied to the 8-line staff.
Here is the project:
Intavulatura.dorico (538.1 KB)