I will try to explain, I'm not 100% sure but this is my educated guess. The BATF will only allow you minimal latitude in modifying MGs, the HK sear will go into any of the HKs that are functionally identical. Most KH long guns are of the same pattern and just use a variant of the same receiver so this is OK. The Sten receiver is similar to the Sterling so the BATF has allowed people to remanufacture Stens into Sterlings as it is not too much of a change. Those are the only two examples that come to mind right now. Fabricating a functioning machinegun from a HK SL8 and a registered sear won't pass their muster. If it would work I, and others, would buy up all the MAC 10s I could lay my hands on and convert them to FN-MAG 58 and other fun stuff that is rare or unheard of in the US (transferable). Does that answer things for you?
I base this on a conversation I had a while back with the guy from Long Mountain Outfitters who once made a device that would allow him to use an M16 DIAS in an AK I think it was. He sent a letter to the BATF about it and he was told that his device that allowed his drop in sear to be used was, in their view, a machinegun. It did not matter that his conversion already required a registered machinegun to work (the DIAS) he needed two machineguns (per ATF definition) to make one gun fire fully automatic.