AFAIK Rejig and IFOEdit support only one title so say bye bye to that.
In terms of free apps that leaves you with DVDAuthor.
MuxMan 0.15+ is a non-free solution. So anyone using your GUI would have to pay $5 US.
As an aside I have my own solution that will take m2v/ac3 pairs (up to 8) and author them using DVDAuthor, with a crappy autogenerated menu. It does support cli.
A prior version which supports 4 pairs is bundled as part of this:
http://forum.doom9.org/showthread.php?t=100230