It wouldn't be very realistic just allowing to call the deicers from anywhere.
Hello.
Maybe it's not the best and most realistic solution, but it would solve the problem.
It definitely would be more realistic than just being able to de-ice at the gate.
Now it's even more important since the holdover times are counting.
We can't depend on scenery developers to add correct de-ice areas. We could be given the opportunity to add something to the afcad to signal gsx than that was the correct spot for de-ice.
Or just treat the de-ice trucks as a follow-me. Have them meet you where you call them. If a user wants realism I'm sure he will only call the trucks at the appropriate de-ice bay. If a user decides to call the trucks somewhere else I'm sure he will not complain about lack of realism.
[btw: that would be a good way to implement some gsx - rescue version! ;-) ctrl+f12 / 8 - call rescue vehicles]