As a noob BiaB user but an experienced SONAR user, I just wanted to note for the benefit of posterity that this does not have to be so difficult.

Unless you specify a custom path, SONAR installers have always laid down the 32-bit application in C:\Program Files (x86), completely separate from the 64-bit installation in C:\Program Files. And the DLLs for the two flavors will be registered independently from the two paths.

So if you have both flavors of SONAR installed to default paths, all you have to do is point* BiaB to the DLL in C:\Program Files (x86)\Cakewalk\Shared DXi\, and Bob's your uncle.

If you don't yet have SONAR x86 installed because you're normally running the x64 version, you can install just the 32-bit TTS-1 - and any other 32-bit plugins you want to use in BiaB - by choosing the Custom installation option and unchecking everything but the desired plugins. (If using Command Center to install, you'll need to enable 'Verbose' installation in Settings to get the Custom option).

* (For the benefit of noobs like myself, DX plugins not present at the time BiaB was installed can be scanned and/or registered with BiaB via the Options button in the 'VST/DX Synths/Plugins' dialog accessed via Preferences or from the Plugins tab in the Mixer window. This is the main piece of the puzzle I was missing).