Well, surprise!. I did get a response from Kodak Alaris in Rochester today by their "Worldwide Photo Chemicals Product Manager." He provided an initial answer and later corrected it. FYI, the first response:
HT-2 was approx. 3.5% acid solution. Indicator Stop Bath makes approx.1.5% acid solution when diluted according to the product instructions. Altering the dilution of Stop Bath to 1:25 (parts concentrate: parts water) would make approx. 3.5% acid solution. We do not know what precision is needed but the hypo test is a visual spot test to be used as a “rough guide”, so it should be OK. [Notice there's no mention of the possible effect of the indicator in the stop bath on the test result.]
The corrected response:
I should revise my answer -
The hypo test contains silver nitrate which is absent in the Stop Bath. Best to mix according to the formula and buy the ingredients (available in small quantity from a lab supply).
So, I'll get the Formulary Residual Hypo Test from B&H (same price as PF but much less shipping cost).
Thanks, all, for your comments.