I’ve just tried experimenting with this, and I agree, we certainly need an option for dotted quarter-notes (which is probably the way you’d be wanting to do the tapping).
However, it works fine if you tap all the 8th-notes, set all tracks (including your new “tapping” track) to Linear Timebase, use “8th-notes” as your base when doing “Merge Tempo from Tapping”, and then change your time signature to 6/8.