There are many ftp clients in repos, (there are even ftp server). A recent one
(http://talk.maemo.org/showthread.php?t=55336) seems also very nice and usable.
You could just download and edit the html file on the n900 localy with any text editor, and then re-upload it afterward. At least, this is how i proceed on a daily basis! And normaly, your html files should not weight more than few kbs, so it's a matter of seconds!