Longtabu total width, fixed left margin

Longtabu total width, fixed left margin

I'm trying to make a lontabu that is wider than the textwidth at a certain
moment. I've tried different things always ending up with a different
problem:
First possibility: If I use the adjustwidth environment the longtabu does
not allign correctly. The left margin stays fixed and the right margin
gets out of the page. This in contrast, if I do the exact same thing to
make a tabu(instead of longtabu), it works perfectly. I think using the
adjustwidth is the best way to go, but I can't manage to get it working.
Second possibility: I've found a solution using the fullwidth package,
that package does the trick but using endhead and endfirsthead is
completely ignored, so it does not help me either, because I need that.
Final possibility: Using the geometry package does the trick, but that
package adds a pagebreak when I start using it, I can't have that. It has
to be directly under some text, that has a narrower textwidth.
Cheers.