This is most definitely not the first time this has been discussed here. From what I remember, I've never heard a technical objection. Concern has always been that if an app could be used under the host OS, then this would reduce funding for the ROS equivalents.
Sadly we are in a different situation now. There is little to no funding for anything and in the current environment if you need to use a particular piece of software, you are going to use it - even if that means using a different OS entirely.
To put it another way - lack of catering for the times when you /have/ to use a non-ROS app is what causes people to leave the ROS environment. If they don't use ROS for work, then ROS becomes something that you boot up once in a blue moon for nostalgia - ring any bells?
On the other hand - if you could use that key app in a ROS desktop, you'd start using all your favorate apps - have !digitalCD pumping out your favorate music and use !Artworks to make your graphics... ROS could be your work environment again...and think of how much of an encouragement that would be to develop apps and utilities for ROS again.
We're only here because we love RISC OS. We just want a way to use it all the time - even when you /have/ to use an app on a different system.