Yes, I had done that already, but nothing seems to happen.

I only see an empty "CodeBrowser" window in TextMate but I don't know what should appear there ...

- In koma-script alternative forms of the document structure commands exist: \addpart, \addchap and \addsec (which generate unnumbered headings) and \minisec. Is it possible to add those as well?

But I will have a look at the LateX and Beamer bundles and try to understand what they are acually doing. Perhaps it is really a good idea to copy the standard bundle and add the koma-specific terms here and there.

No need to copy the entire latex syntax. You can just tell your syntax to import the latex syntax. In fact the latex syntax includes the tex syntax this way. The last pattern says:

{ include = 'source.tex'; },

You'll just have to do a similar thing with latex:

{ include = text.latex'; },

Then before that you add your own things. If the other maintainers don't object, and if there are enough users using koma, we could add those as well in the official syntax. I just don't want to add support for any little things that any bundle offers. Also, the idea of TextMate is that you customize things to your liking. For instance in this case, you can just add them to your local copy, and then share it with people. For extra sectioning commands, all you have to do is change the line:

begin = '((\\(?:(?:sub)*section|chapter|paragraph|part))(\*?)(?:(\[)[^\[]*?(\]))??(\{))';

to 

begin = '((\\(?:(?:sub)*section|chapter|paragraph|part|addpart|addchap|addsec|minisec))(\*?)(?:(\[)[^\[]*?(\]))??(\{))';

that should do it. Let me know if you need any help with any of the other modifications needed.

Thanks for your patience with us TextMate newbies,

best

My pleasure. Been there, still am in many ways. 

Matthias