mathdigits.mkiv is a typescript. Here is a simpler version using defintfontfamily and definefallbackfamily, which doesn't work in the recent beta (but worked well back in January).
\definefontfamily [minionmath] [rm] [Minion Pro]
\definefallbackfamily [minionmath] [mm] [dejavusansmono] [range=digitsextendedarabicindic,offset=digitsnormal]
\definefallbackfamily [minionmath] [mm] [dejavusansmono] [range=digitsextendedarabicindic,offset=digitsbold,tf=style:bold]
\definefontfamily [minionmath] [mm] [TeX Gyre Pagella Math]
\startttext
\switchtobodyfont[minionmath]
\startformula
{\tf xyz} xyz {\bf xyz} {\bi xyz}, 2+3, {\bf 2+3}
\stopformula
\stoptext
With beta version 2017.01.17 17:37 it used to work and change the digits to Indic inside math. With beta version 2017.04.27 01:00, the digits do not change to Indic. However, everything in math changes to dejavusansmono (bold, because that was the last fallback definition?).