There's a huge difference between an excommunicated king that already established his authority, and a excommunicated claimant among other that aren't (Conrad V, Louis IX if he claims his brother title)
Given the reputation of Manfried as murderer of Conrad IV, the fact he tried to pass Conrad V as death already, trusting him regency would be...hazardous at best, sucidial more likely.
There's no such thing as "German Princes" as one political entity : some were opposing Capetians (rather than "French", nationalism isn't established yet), other supported them along their interest, and other were more or less benevolently neutral (remember that Louis IX is then the more powerful sovereign of western Christianity, and beneficy from a really good reputation and prestige).
Furthermore, HRE was quite troubled then : no Emperor up to 1275, kings and anti-kings popping around...I'm not sure you would have an unified and efficient answer there critically when the Capetian king refrained to launch campaigns against HRE, being more or less neutral then within Pontifical/Imperial conflicts.
In fact, I could see Louis IX trying to support Conrad V's claims if sort of an alliance is made.
If the a king or anti-king try to support Manfried for...some reason, in their troubled situation, it would be hazardous as well.
For Manfried rallying HRE, it's a big no : he doesn't have legitimacy, prestige or ressources for doing so.