Do you think that you should look into why your geting the dust / dirt ?
What is your spray room like ? Do you clean it often ?

Do you have "dust bunnies" hanging from the ceiling of your spray room.

What about the type of ventilation ? Is the fan strong enough to suck out the dust in the air ?

Are you using an air hose to blow the dust off prior to each coat of finish ? ( Assuming that your cutting between coats)

Can you do a "spot" repair ? Do you mean after your all finished and it's no longer in your shop ?
If it's still in my shop and I find dust or what have you . I would cut it and spray another final coat.
If your talking about doing it on site, it's much more involved and in my opinion will never look as good . Also the fact that it's a semi gloos doesn't help the situation. The higher the sheen the harder to do a repair or touch up.

I would work on why your getting the dust and try to eliminate it from the start.Then ya don't have to worry about touch ups.