I already got your intention I think
If we refer to e.g. skins of cars it is a real good proposal to just pack the skins as an update to the cars mas files itself in order to save some download bandwidth or just to keep the update rather small.
Sad but true. I think it is not documented clearly enough, missing examples and the overall documentation seems to be somewhat outdated ...
Just an impression, but I have the feeling I am right.
I am not sure about the mft file and if we can create it on our in advance (I don't think so). We could - I would say - if we know how to create signatures (not sure if there is an option to modmgr for it).
Just a simple approach: do we need to say it is an update? E.g. if we keep the name and we are just increasing version won't it be stored in the same directory? (while writing this it comes to mind that base versions are not kept if there is a newer one ... compare it to car upgrades)
I was working on a rF2 Server Manager (powershell based) and it is hard to implement a certain logic, especially how to handle content and how to set up e.g. mod files (pkginfo.dat ones) for creating mods.
So I stopped ...
you know what...
I tried again this evening just for fun.
the issue seems directly linked to the fact that modmgr doesn't add BaseSignature=value in the mft before packing the .mas. When you create the 2 rfcmp and install them, just adding the basesignature in the 2nd directory mft does the trick and modmgr show it as a unique updated mod, removing the line shows it as separated mod, ever we don't know the syntax or the tool doesn't take this into account. Seems strange if MAS does it but not modmgr.
So I will try to hook it in some ways.
I'll let you know if I make any progress.