Just picked up a (used) dedicated upper.  PSA receiver, CMMG bolt.  I haven't fired it yet.  There is a dust cover on it, but no forward assist, just the empty hole.

1) With nothing plugging the hole, am I going to get a lot of combustion residue blasting past my face every time I fire a round?

2) If "yes", what can I use to plug the hole, either temporarily while I zero a new scope, or more long term?

For the purposes of the long term solution, I don't see changing this upper to another caliber, if that matters.

Schuster forward assist plug

You can use duct tape to temporarily (or permanently) plug the hole.

.45 ACP empty. A hole can even be carefully drilled for a roll pin.
A wrap or 2 of tape if it's too loose.

It looks interesting, if done carefully.

Attached File

Last I looked, there were at least four companies making and selling fill plugs for the FA hole.  An internet search should display them.
My personal experience from a couple years ago with a stripped upper that I am using for a dedicated .22LR was, that at the time it was cheaper (when shipping was added in) to buy an upper completion kit with cover, rod, spring, retention c-clip, FA, and roll pin than to buy a fill plug and roll pin!  Probably not true today, though.
MHO, YMMV, etc.  Be powerful; be well.