[[http://4-ch.net/ 4channel]] would create a link with text "4channel" that goes to http://4-ch.net/. It seems easy enough to implement: replace [[ with <a href=", replace the space after URL with ">, and replace ]] with </a>. It's based on the Wikipedia/ED style of linking and it's a lot more convenient than BBCode. You could also use [ and ], but those are sometimes used in plaintext which could be an issue.
Rather than inventing a new format that sort of looks like something else but isn't, why not use a format that already exists? Considering Wakabamark is more or less Markdown-based anyway, it would probably make sense to use Markdown's link style, if anything at all:
[Wakaba and Kareha support board] (http://wakaba.c3.cx/sup/)However, in the years I've used Wakaba/Kareha boards, I've rarely felt encumbered by the link formatting. "blah blah blah (http://blah)" works just fine, and More options -> HTML format is there for the rare case when it isn't.
I decided against bringing over this feature of Markdown mostly because I judged that it is clearer and more in the spirit of casual text on a discussion board to include full, visible links. Discussion is not hypertext, and links work better when they stand clearly apart from the text.
And there is HTML mode if you absolutely, positively have to have inline links.
PS: If you are a programmer, >>1, never ever implement any kind of formatting in the way you described. It is a horribly unreliable and insecure.
>>3 [[http://example.com/"onClick="evil click me!]]?
That's one thing. Even if you fix that, there might be strange and unexpected interference when you mix several types of markup in invalid ways.