Wait, do you mean to say that unpregnant male Sims will be unable to use the clothings that these are based on if one puts this modified mesh in? I'm kind of confused. Please expound. What exactly do these files do -- add a mesh, or modify one? And why does it make it "useless for regular wear"?
Unless there's been recent modding progress I don't know about, there's no way yet to add a mesh to the game, only to override an existing one. So once you have the big belly PJs in your game, each and every YA-A male wearing PJs will have a big belly, pregnant or not. This is the trade-off: for every pregnant outfit you lose a non-pregnant outfit.
I'm not positive how female baby bumps are done in the game, but there seems to be a pregnant morph since the same clothing is wearable for both pregnant and non-pregnant women. Have to explore more.
I see that someone on MTS and someone on Custom Sims 3 have enabled the adult maternity clothing for teens. Is doing the same for male teens plausible?
I haven't looked at teens yet, but most likely they can be done the same way. Balloon Head did it by enabling adult female clothing to be wearable by teens, with that clothing presumably having the pregnant morph built in. (I don't know what he did about the size difference, or maybe the game scales the bodies down?) That won't work for males unless you want your teen guy to suddenly have a female body when he gets knocked up!
Of course, the Holy Grail of Sim mpreg is to add a pregnant morph to male clothing so we can have both normal and pregnant forms.