Andre Engels wrote:

What, if anything, can be done in Wikipedia with texts and pictures that fall under the Open Content License (either in original form or one of its restrictions)?

One of it's restrictions? The Open Content License is pretty restrictive as is. Perhaps you mean the Open Publication License, which is a free license, but has two options, which, when exercised, make it non-free. It's probably best not to try to merge this stuff, unless you can get the original author to add the GFDL as an adjunct, as it would be too much of a headache for the vast majority of contributors.