Simply put, Manga Video just got the rights to Macross Plus directly from Big West and HG can't do anything about it. The "excuse" made by HG is a red herring, saying that Saban hired off HG's staff off. In fact, Saban was already well established back in 1985, with "Macron 1" (a spliced anime of GoShogun and Strungle) and they already have some staffers/voice actors work on Saban time to time since that point.
I can answer to that because I own the Macross II RPG books, in which they are the first official Macross-related RPG product. B)
All of the books have Big West and US Renditions on the copyright notice and US Renditions logo on the back on some of the sourcebooks. The Siembiedas stated that they got their license through US Rendtions and Big West, though they didn't update to have Manga's name back in 1994-1995 when they were still in print. Also, Harmony Gold didn't do anything to stop it nor crosspromote it with the Robotech RPG.
Hell, Kevin Siembieda stated (Macross II RPG book, pages 5-6) on the introduction that Macross II should be treated as a Macross-only work and not to be confused with Robotech. He even delve into the history and license rights on who owns Macross-related animation. Also, he said he pursued the license because he was a mecha anime fan, hence he got the RPG licenses to Robotech and Macross II.
Aside from using all of the Macross II source material for the RPG and can't expand it, Palladium Books can't get the licenses to Macross-only stuff because Big West is asking too much on the license fee and PB can't pursue this at this time.
If this is the hole HG can't cover, I hope someone just get a lisence directly from BW. Maybe Manga can release M7 series in DVD, eh?
.......So I guess Tokyopop won't have any problems getting the M7 trash.