Set a value on a separate object then check that value directly after the collision check, if the value is set set the flags and reset the value. I use this trick all the time to get around the object selection.
I think a fastloop is the way to go. This is what I always do:
+ Object_A collides with Object_B
-> Start loop "Set_Flags", 1 time
+ On loop "Set_Flags"
-> Object_A: Set Flag to ON
btw: If you happen to be using it in your project already, the "Select Object" extension can do exactly what you want, without the need for the extra event - although it's obviously overkill if this is all you're going to use it for.
It's a shame you can't say something like:
+ Never
OR (logical)
+ Object_A collides with Object_B
-> Object_A: Set Flag to ON