I assume then it would require a manual check of each order?
yup
We're trying to work within the limitation of what we have now.
On a serious note, as a e-commerce professional, I'm surprised that it would have to be this way. At least a while ago you had a system for linking the main-site account to the forum account.
All you'd need to do would be to implement a voucher collection page, probably one with a unpublished route only disclosed on the forum. You can then make that page grab the forum account id from the account if the user is logged in, allowing them to see a voucher code locked to one per account upon meeting the criteria.
Either that or if you have an event for voucher claim, with a thread in the forum disclosing the voucher codes, you could potentially just catch that event and do all the leg work in the event for validating the forum account.
In theory it should be very doable, admittedly I have no idea what you've used to implement your systems, but could imagine something like this is entirely plausible.
If you do have extreme levels of technical debt, I strongly recommend seeing to it though, having a custom scaleable automated E-commerce solution is going to become more and more valuable going forward as drop shipping, out-of-the-box solutions and Amazon eat most of the remaining little guys.