Huh... Latex sure goes deep. I took a quick look at the manual for floatrow (72 pages) and it seems like it will do the job as you say, I'll try to figure out how to use it tomorrow...
it actually worked quite well! It's not perfect as the caption now sticks to the left side of the page instead of the table , which means that if the table isn't as wide as the text area the caption will be too much to the left (as the table is centered and has some space on both sides).
I'm new to latex and I'm using lyx right now. Apparently it's not possible to create captions for longtables the "normal way" because it's not a "float" or something like that, so I have to use this dirty workaround and make a new line in each table and place an ERT line in there. Now I ...