What's a good sauce or spread for bacon wrapped pork loin? Was thinking some kind of gravy or mustard spread.

How about figs, dates or plums? I think the sweetness would pair well with the smoky flavors from the bacon and the following recipes have vinegar for a hit of acid.
https://food52.com/recipes/27039-pancetta-wrapped-pork-tenderloin-with-date-jam
https://food52.com/recipes/308-plum-sauced-pork-tenderloin
https://food52.com/recipes/14391-roasted-pork-tenderloin-with-fresh-fig-sauce

I like your idea of mustard. You could start with the recipe below and doctor it up, as suggested by Marsha, with honey/maple (maybe brown sugar)...
https://food52.com/recipes/19410-roast-pork-tenderloin-with-sauerkraut-red-potatoes-apple-cider-mustard-sauce

maybe some kind of mustard sauce with honey or maple syrup, or peach jam.