[Natch]

With other characters, you will need to test adding those characters into the regular expression test (bolded)... I've added your # symbol as an example...

RewriteEngine on
Options +FollowSymLinks
RewriteRule ^~([a-z0-9\ -=#]*)$ forumroot/member.php?username=$1 [L,NC]

and if you get a 500 Internal server error, you might find you need to escape the character you want to add... (eg look at the way the space character is escaped with the "\" beforehand)...