The Hot Pepper said:
When you prep the peppers for the sauce you can cut the placental tissue and membranes (white ribs) out from the inside with a paring knife. This is not a problem when making small batch but takes time. All this tissue carries heat and no flavor except bitter. You will achieve what you are after.
^ What he said. It'll reduce the heat tremendously.
Personally, I like the "grow other varieties" suggestion. You can never have too many plants (he said, half tongue-in-cheek). Here are some ideas. Some of them, I haven't gotten pods from yet, so I don't know what the flavor is like, but they're all sweet or mild chinense varieties.
NuMex Suave. Whereas the typical habanero is in the 250,000 to 300,000 SHU range, NuMex Suave Red clocks in at less than 800 SHUs, and NuMex Suave Orange isn't even half that. As mild as they are, for purposes of a hot sauce, you'll probably find yourself wanting to blend in a few regular habs.
Venezuelan Tiger. While I wouldn't compare the flavor to a hab, it's a sweet chinense variety with a very good flavor that I routinely blend into my sauces.
Aji Jobito. Another sweet chinense. Haven't gotten to try it yet, although I'm growing it, so I don't know whether the flavor is anything like a hab. Everyone seems to rave about it, though.
Aji Dulce Margariteno
Sweet Habanero
Sweet Scotch Bonnet
Sweet Datil
Stuffing Scotch Bonnet
Star of Turkey: supposed to be about 1/3 the heat of a habanero
Tobago Treasure
Tobago Treasure White. Just got my first pods the other day. Absolutely delicious, and milder than a hab.
Trinidad Perfume. Personally, I wouldn't bother with this one. It has no heat at all. The flavor strikes me as minerally and somewhat tart. They're okay for eating, but I probably wouldn't use them in a sauce.
Maldivian Heart
The various Aji Dulce and Seasoning Pepper varieties (I'm partial to the Grenada Yellow and St. Lucia Red seasoning peppers)
Aji Cachucha. Nice flavor, no heat. You'd want to blend this one.
Valencia Market. Excellent flavor. Hot, but not overwhelming.