Dear all,
Many thanks to everyone who replied on- and off-list!
In addition to your suggestions, I have also been looking at the section 'Transfer of present classes' in Oberlies' Grammar of Epic Sanskrit (2003: 189-202), and Lanman's On multiple presets and on transfers of conjugation in the Sanskrit verb system (JAOS 13 (1889) XXXVI-XXXIX). It is interesting that secondary -aya-formations are often excluded from such considerations, probably because they are automatically assumed to be causatives rather than alternative (simple) present-tense forms.
Thank you again, and I hope you all have a very good week ahead,
Antonia