There is no errata published for Firing Squad, so there was nothing to miss.
The asterisk'd exception for hold-outs was there because Pistols was listed, and hold-outs is being treated differently than other pistols. So there's no grounds to compare Launchers, here.
Barring future errata, we're in a position where Launchers are not granted an allowance ergo there is none.
However, no weapons that use other than Close Combat/Firearms are listed at all. Not just no Launchers but no exotics of any kind. Also, no bows/crossbows. Technically, they're all not mentioned and therefore can't mount modifications, but in certain cases some seem like they should plausibly apply. You can use GM discretion as to whether certain unusual weapons are compatible with this or that modification. E.G. Smartgun on a Crossbow? I say no. Laser sight on a crossbow? Ok... maybe. That's much more reasonable, at least. Personalized grip on a crossbow? Ok, that actually makes perfect sense.
If you are going to extend modifications to these kinds of weapons under the Aegis of GM discretion, you'll also be responsible for ruling on how many slots they get. I'd recommend looking at the extant list and giving a # based on weapons of comparable size.