That is possible yes, to fill the gauge rather quickly in the final moments. However the gauge can only be filled by spraying over other ink or unmarked territory (and we're at the end of a match so the likelihood of unmarked territory is pretty low), thus you must be in an acceptable position since you would have been penalized on the meter for being splatted.
If you're already in an acceptable position, while it can hold true to turn the tide after the clock, the fact the other team would allow something that spreads about 15~ mg of ink to make them lose is embarrassing.
I'll rephrase then, the weapon -and- the enemy is dumb.
I'm usually able to fill it like that, but... Nerfing inkstrike as a whole because the Aerospray is stupid wouldn't be fair to the other sets that use it.
Anyway, its...tough to say whether or not it should be counted. I mean, in other games when time is up, time is up. Unless something stops the clock I really don't think it makes sense for something to count after the round is over. Correct me if I'm wrong, but in most shooters once time is up or whatever objective is achieved. As an example, in CoD (why did I just type that) If you're in the middle of getting shot and the match ends, you and everyone else is made invincible because its over. You had your time in during the round to do something, why should some gimmick be a work around?
Same thing in fighting games, if I throw a projectile and the round ends, you won't be injured, because the round is over.
Look at it like this, sitting on the inkstrike is dumb. We can agree on that. But if you're in a situation where there is 15~ seconds left on the clock and you know you won't be able to fill another bar then waiting to send it out until after the round finishes would be a smart idea. Its impossible for them to contest it now, and so long as you have enough prowess to back yourself up and not die, it'll work. That is just sort of dumb. Not to say I haven't done it many a time, but it just feels so ehh.
Its kind of like the round ending in a boxing match and you throw in one last hook after the bell rang.
Just my 2cents