Dear Bhante Sujaato,

> The curious thing is that this compound, the reading commented on,
> does not occur in the sutta, here or anywhere else. The sutta just
> says 'ekaayano ayam'. Nor does the particular comment occur in the
> Majjhima or Digha commentaries.

I found that 'ekaayanvaaya.m' occurs in the Thai Budsir IV version of
the sutta and, furthermore, it occurs a total of 10 times in the same
Mahaavagga volume of the Sa.myuttanikaaya according to the search
results (not counting the possibility of more in the peyyala
sections). The PTS reading in the sutta also has 'ekaayano ayam' but
there is a footnote for a reading in a Burmese ms in Paris: -nvaayam.

The .tiikaa comments on the sandhi:

367. Ekaayanvaayanti sandhivasena vutta.m o-kaarassa va-kaara.m
a-kaarassa diigha.m katvaa. Aya.m kira sa.myuttaabhilaapo, ...

'Aya.m kira sa.myuttaabhilaapo,' (?? this, it seems, is the Sa.myutta
wording) as if the commentator thought the sandhi-form unusual.

Best wishes,
Jim