I doubt initially it would be available on reward seats, but I am just speculating about that. The fare construction on reward tickets is different to commercial tickets, and the latter is a transferable skill, someone who does commercial ticketing for BA and / or IB can generally do the same job with any Amadeus airline or indeed work in a travel agency. Redemptions attract internal rules which aren't so transferable.

The usual way Plusgrade works is that they prepare a template letter for potential customers, the airline sends that template to customers who are in scope for upgrades, but the reply / links in that letter go to Plusgrade's auction page. I say auction but it's absolutely nothing of the sort. On that page you make a "bid" for an upgrade between a minimum and maximum set by Plusgrade. Plusgrade typically has some inventory under direct control, and after a few days, couple of weeks, they confirm your bid if it is within the price range that Plusgrade wants to operate. Then there is a like a clearing process, bids which are not successful in that first sweep can get any last minute inventory for seats which otherwise will be empty, so lower bids may get accepted say 2 days to departure. These 2 passes are invisible to the passenger since they only get two emails from Plusgrade - one confirming the bid, the other giving the outcome of the bid. Since that second email can come early or late the process becomes opaque to the consumer.

Shorter answer is "don't ring us, we'll ring you".