The issue I have is whether or not I can use Opuntia pads in place of other low pH ingredients and test the pH of the finished product a couple days later for accuracy and make it using a BWB.

Can you? Sure, in theory it should work and obviously commercial canners do it.. But it would be more like making ripe tomato salsa than making green tomato salsa since they at least have a higher pH. Still it would contain no low acid ingredients except the added acid.

Should you? That's up to you. As you say, no known tested recipes so it will be a do at your own risk thing. Not recommended, just your personal experiment.

I do know that you have to get it below 4.6 and that using bottled lime juice will work better than vinegar for doing that as well as compliment the taste more. You can also use citric acid. But testing just the once a couple of days later probably won't be enough (NCHFP tests repeatedly to plot out when the pH begins to rise and how much) so I'd test again before consuming.

Proportions of the ingredients is the real problem. If I were to try it I might be tempted to start with the Choice Salsa recipe on NCHFP and see where that gets you pH wise.

Dave

PS: of xourse freezing rather than canning would be the best option

Here is a link that might be useful: Choice Salsa

@ nancedar....
As you have already pointed out, there is only one issue here : pH( or acidity level). Since you already have a reliable pH meter you can do it as accurately as anybody can.

Measuring the pH of the solution is a piece of cake for you. Now come the question of HOW MUCH the solids can increase the pH(reduce the acidity) ?. There is a simple way: Take the most extreme scenario; that is assume that all of the solids are converted into a solution(after canning), with a pH of 7. Then figure out what the pH of the present solution should be to compensate for that situation.

Example: lets say that the ratio of solids to liquid is 1:1 and its pH is currently measuring 3.
Take 100 cc of that solution and double its volume by adding 100 cc of water. Now measure you pH. This will represent the worst scenario case.

I believe I will do some sampling (fresh) and take the BWB'd product to my state's food safety lab for testing. I was considering the taste of the opuntia pads which may be more vegetable flavor than the tart flavor of tomatillos or tomatoes and that was what I was asking, I think. But, with a zillion pads to play with, I can do a lot of tasting before I find a salsa that pleases. Thanks for your input.
Nancy