Try using string8 instead of string_interface_i.
And if you're feeling bold, or just plain lazy, you can get away with converting a whole mIRC command string
//mp3title [%artist% - ]$if(%title%,['['%album%[ #[%disc%/]$num(%tracknumber%,2)]'] ']%title%,%_filename%) | /mp3tme %_length% | /mp3file %_path_raw% | /mp3rate %__samplerate% | /mp3bitrate $ifgreater(%__channels%,2,%__channels%0000,%__bitrate%) | /mp3channels %__channels% | /mp3seconds %_length_seconds% | /mp3play
or a whole spam script URL, ready for wininet, complete with lazy $replace() escaping (note use of single quotes to enclose brackets that I want to send, percent signs, conversion of spaces to plus signs after escaping existing plus signs, and the empty pair of single quotes that Tagz turns into one single quote)
http://127.0.0.1/submitdb.php?title=$replace([%artist% - ]$if(%title%,['['%album%[ #[%disc%/]$num(%tracknumber%,2)]'] ']%title%,%_filename%),&,'%'26,+,'%'2B,' ',+,#,'%'23,'','%'27)&len=%_length_seconds%&user=kode54
I'd have written a configuration dialog for it, but most would probably find it a chore to assemble URLs for their favorite message board signature music spamming service, not to mention that your password would have to be somewhere in that field. Not to mention that I never felt like begging for a tester slot at WinampDB, or writing the code to fetch the submission password. Instead, I wrote my own insecure spamming system, which I even used here for about a day.