Software AG claims to be “independant” company, as such they (thanks god) offer the software for the mayor OS’s, however some components such Designer/Developer still misses this important key (They only work on Windows, yiaks! ).
With this in mind, creating a “Outlook plugin” is shown as an example from which you can implement your solution, else I woould fint unfair not doing the same for Lotus Domino, or the nice and lovely Linux evolution.
You can always purchase professional services assistance to implement solutions based in your needs, if your company does not have the proper resources (Yes I know you do not want to pay any extra cost).
The attchment solution built in MWS, lets you add documents as part of task context and you can open/modify and complete the task. (I’m sorry but a link to a document on webdav or jsr container is a valid solution).
I do not understand very well how you expect to modify something in the “Fly” without using any kind of storage. As I mention you can open directly a document sitting on a container, modify and save changes which gets persisted on same container. When you complete the task the document is ready to go for the next step.
Aside from the built in plugin, the other solutions exist already out of the box.
Maybe other products has nicer approaches, but more weakness in other areas.
All is about balance pros/cons and decide which software suits better for you.
Maybe you should consider using this other software you mention? just balance your expection, price and results.