I received seven Lancer magazines this morning (5 10-round and 2 20-round) and was able to fit a .450 Bushmaster follower to one of the Lancer magazines. I still need to clean the follower up a bit but it wasn't technically difficult to do and it appears to work just fine. I plan on putting the Lancer 5.56 follower in the BM magazine and will repeat this process four more times for the other .450 BM mags that I have.
The feedlip spacing is different for Bushmaster and Lancer magazines so the top round rides a bit higher in the Lancer magazine
BM rear > .424" front > .436"
Lancer rear > .450" front .465"
Also, the angle of the feedlips is different with the BM feedlips being virtually horizontal relative to the cartridge. As the photos show below, the feedlips do affect the presentation of the cartridge to the chamber which will be more exaggerated as the number of rounds in the magazine increase due to increased spring force. The feedlips limit upward movement of the cartridge which is independent of the follower. The BM feedlips flex whereas the Lancer feedlips appear to show no flexing at all. Also, I don't see the need to remove material from the front of the Lancer magazine since the cartridge easily clears the tube during feeding.
Bushmaster follower on left, Lancer follower on right
Bushmaster magazine top, Lancer magazine bottom, both with BM follower
Lancer magazine left, Bushmaster magazine right, both with BM follower
Lancer magazine left, Bushmaster magazine right, both with BM follower
Lancer magazine left, Bushmaster magazine right, both with BM follower