Just a comment,
1) You are in webmail and mark a msg as white or black
2) If you see the horde database, table horde_prefs you can select out the whitelist o blacklist of every user in the database
something like mysql
> use horde
> select pref_value from horde_prefs where pref_name = "whitelist" into outfile "/tmp/whitelist.txt";
3) With the withelist.txt then you can cut to get the e-mail address
something like cut -f2 -d"\"" < /tmp/whitelist.txt
4) With all this stuff then probably you can insert the e-mail addresses into the spamassassin doing something like this
for b in `cat /tmp/whitelist.txt`; do
db spamassassin setprop wbl.global $b White
done
5) If you can insert this in crontab, then probably you can connect webmail whitelist/blacklist with spamassassin whitelist/blacklist
The only thing I can see with this is there is no black or white lists per user. The lists are general.
I hope can help
Regards