I'm sad I didn't find the article “Incorporating Guile into your C program” before. It would save me a lot of time. Although the author annotates the article as “a old howto I wrote (very probably terribly out of date at this point as I gave up on Guile a long time ago)”, the article is still correct and very useful (especially in the GC part).
1 October 2005, update
The document is inaccessable today due to problems with the server. Instead, the document can be downloaded from Unofficial Guile Docbits.