From the radius.inc and captive portals index.php file it looks like the following should work (Nomadix attributes):
[Vendor = 3309 (Nomadix)]
ATTRIBUTE Nomadix-Bw-Up
ATTRIBUTE Nomadix-Bw-Down
Imagine you would add these to your radius radreply table, e.g.
insert into radgroupreply (groupname, attribute, op, value) values (“slowspeed”, “Nomadix-Bw-Down”, “:=”, “1024″);
not 100% sure on this though - would be good for someone to confirm this.