yep, i do this myself with a small tweak to the stock code ( paypal_ipn.inc.php )
i set a custom user color and change their title right after it sets their group
kinda like:
$q = "
update {$config['TABLE_PREFIX']}USER_PROFILE
set USER_CUSTOM_TITLE='n00b Sponsor!', USER_NAME_COLOR='#0af'
where USER_ID=$user_id";
$dbh->do_query($q);
so you could either modify the code like that ( probably different title ) or you could appropriate the SQL and save that as a query in Database tools.
3rd option is to make a custom island ( not displayable ) that runs every so often and does that query, but on a group basis to swoop in and ensure everyone is setup properly
I use 'housekeeping' custom islands all the time for this kinda work.