Forcing Buffs

I know you could make an ability that has no effects and all it does is cast a buff on a unit, but that takes time + the spell would only be effective for a certain time period.

Is there a way (I know you can't do it in GUI triggers), to force a buff onto a unit and then remove it after your choice of seconds later?

It has to be possible since it seems everything else is... maybe i'm missing something but I can't find a way to stick a buff onto a unit.

The dummy caster is your only option, even JASS doesn't offer ways to add buffs to units.
If you set the duration value of the buff to the max integer value for the dummy ability you'll have total control over the time period the buff stays on a unit because you can remove it via triggers.
Also a dummy caster doesn't actually take a lot of time, simply set the mana cost and cooldown to zero, and you'll need only 1 single caster unit in your map. Shouldn't be any problem, has been done before.