Koumas came off the bench on the hour mark and delivered the equaliser in the third minute of stoppage time, nodding home a cross from Neco Williams to rescue a draw after Ghana substitute Caleb Yirenkyi had given the Black Stars a 68th-minute lead.
The goal arrived during a match marking the 150th anniversary of the Football Association of Wales, with the home side wearing a special retro red and white kit for the occasion.
Bellamy has been deploying Koumas as a striker in training and in recent friendlies, a deliberate tactical decision driven by Wales's shortage of natural number nines in their development pipeline.
"He is a nine because we don't really have any nines coming through and we do have a lot of wingers, brilliant wingers," Bellamy said.
"He could definitely be a huge asset for us on the wing but, due to our lack of nines coming through the system, we've used him a lot in training that way and he's really been electric."
Koumas has spent most of his professional career operating on the left flank. His loan spells at Birmingham City and Stoke City were predominantly in wide positions, as was the majority of his time at Hull City this season — the Championship side won promotion to the Premier League through the play-offs with Koumas as part of their squad.
Wales's wide options are extensive. Daniel James, Harry Wilson, Brennan Johnson, David Brooks and Sorba Thomas all compete for wing roles, leaving Bellamy with a genuine positional vacancy through the middle that Koumas may be uniquely placed to fill.
Bellamy was effusive about the player's potential while acknowledging the work still to be done.
"There's a lot to like about Koomy. He's such a young player and I'm over the moon he got his goal because he's deserved it."
He added: "When he came on today, he needs to be a little bit tidier but that's normal. I see him as a real future player for Wales who's going to have a big say. I'm delighted for him."
The goal holds additional emotional significance. Koumas's father, Jason, played for Wales in the same era as Bellamy, making Lewis the second member of the family to find the net for the national side.