It is possible to join 725 to a stainless dropout, you just have to think about it a little more. The diameter of the stay is more likely to cause a problem, but below is a 19mm 631 chainstay, joined to a stainless paragon rocker dropout (their slider would work exactly the same).
Dropout to chainstay joint blasted by 18bikes, on Flickr
I have achieved this using a plug brazed into the tube (using brass, the line around the tube) that is then slotted to fit the dropout so it can be silvered together. The issue is that the types of silver most framebuilders use isn't designed for filling large gaps (hence the plug) and is also incredibly expensive, so you don't want to use loads of it. The type of brass most builders use can't be used on stainless.
This type of joint allows virtually any material and diameter chainstay (or seatstay) to be joined to any design/material of dropout.
Here is the finished rear end with the seastays done in the same way, although smaller diameter tubes
18 Bikes 02 by 18bikes, on Flickr
I'm not touting for business as we don't currently offer fillet frames but my suggestion would be to find someone who can build the frame you want, using the parts you want. If you want a tig frame though, drop me a line