Sounds crazy, I know, but is it really?
I think not. Surely you could figure out some way to build into 1P (MAS) the capacity to create one, and only one, unique software license code that is valid for 1Password 3.5.
Individual license? Family license? I dunno. It's commonly argued that a MAS license is more like a Family license than an individual license, but Agile charges the same for 1P (MAS) as it does for a single-user 1Password 3.x license.
Why a 3.5 license rather than a full-fledged 3.x license? Maybe because 95% or more of Macs that can run 3.6 and up are fully capable of running 1P (MAS). And maybe because I just posted something on a thread in one of the 1P/Win forums about creating a special version of 1Password 3.5 that can be activated with 1P/Win license codes. But there are lots of options.
p.s. Bundles (MAS + Windows, MAS + iOS, etc.) are left as an exercise for the reader.