Thank you for the answers. The only remaining thing, that bugs me about those rules, is that apparently an airborne aerospace unit can TAG a ground unit, but usually can't spot it for indirect fire with Semi-Guided Missiles.
As I understand it to perform an indirect attack with Semi-Guided Missiles, and get a benefit for TAG carried by an airspace unit you would need three units:
1. an attacker firing the missiles,
2. an airspace unit carrying TAG (which can't act as a spotter unless it also has a Recon Camera and is flying exactly at altitude 5, where it can use both),
3. a spotter (a ground unit, or another airspace unit equipped with Recon Camera and flying at altitude 5 or higher), which may or may not be equipped with TAG .
Not only would the spotter would have to tell the attacker not only where the target is, but also someone would have to tell the aerospace unit which unit to TAG, and the attacker - when to fire, so that the missiles will arrive at the target, while it is tagged. It is perfectly doable rules-wise, but thematically seems like a lot to coordinate between three units over a span of 10 second turn, even if computers are involved to automate spotter's tasks based on data from spotter's range finder, and whatever devices are used to establish spotter's, attacker's, and TAG carrier's relative positions and orientations. Wouldn't it be easier to create a system, that can establish positions of the target, and the LRM luncher relative to the airborne TAG carrier in real time, essentially turning the airspace TAG carrying unit into a spotter?
As a side note I can't imagine indirect fire in Battletech without some computer system calculating firing solution based on data from the spotter (and information about the attacker's position) in teal time. The "man with a map, binoculars, range finder, and a hand-held radio", sometimes described when people ask, how spotting works in Battletech, just couldn't do the job during a 10 second turn, especially if all involved units constantly move and fight (as they often do in Battletech) without some computer or computers doing all calculations and most of the communication. Looks to me like Battletech C3 computers (at least C3 slaves) are just a perfected version of artillery computers carried in a pocket by at least one man in every infantry platoon at least since the birth of LRM technology. ;-)
It seems that whoever wrote the rules allowing the aerospace units to carry TAG, didn't think about using it for anything else than guiding homing Arrow IV missiles (which, as far as I understand the rules on p. 181 and 354 TO, don't require a spotter - only TAG) or laser guided bombs (p. 246 TW), and didn't even consider using them for Semi-Guided Missiles. Seems possible considering that both TAG, and Arrow IV appear in Polish edition of Battletech Compendium from 1996, but both semi-guided missiles, and laser-guided bombs aren't there (I guess they were added to the rules later, and in case of semi-guided missiles by someone who at the time didn't consider, or remember, that TAG can be mounted on aerospace units).
It is probably not a high priority problem, but something to keep in mind if you ever decide to clarify or change the rules regarding semi-guided missiles, spotting and/or using TAG by airspace units.
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By the way - as I understand it a landed drop ship is still considered an aerospace unit, so your ruling, that it can spot for LRM fire contradicts current errata for p. 111 TW regarding spotting by airspace units. At the moment rules prohibit spotting for LRM fire by any airspace units regardless of their type, and whether they are landed or not. Looks like something that should be changed by another errata, unless there is the exception for grounded spheroid dropships elsewhere in the rules, and I just missed it.
