No, I don't believe it's worth the complexity to try to provide multiple versions of modemmanager simultaneously in a single Ubuntu release.
I don't understand what you mean by "great regression potential". modemmanager 0.7.990 was released 2 months ago but it's not marked as "stable" yet; however GNOME 3.8 will depend on it for the next GNOME stable release due next month. We already package the latest development snapshot of networkmanager which is the most critical reverse dependency and should work fine with the new version.
No, I don't believe it's worth the complexity to try to provide multiple versions of modemmanager simultaneously in a single Ubuntu release.
I don't understand what you mean by "great regression potential". modemmanager 0.7.990 was released 2 months ago but it's not marked as "stable" yet; however GNOME 3.8 will depend on it for the next GNOME stable release due next month. We already package the latest development snapshot of networkmanager which is the most critical reverse dependency and should work fine with the new version.