Only VIP members (50 btc), Donators (10 btc), Global moderators, and admins can display custom titles.
That's not quite true.
VIPs, global mods and admins can set/change their own custom titles, but everyone can display one (eg get one assigned to them by theymos).
An example for that would be
Blazed, who's neither VIP nor involved with moderation, but holds the custom title "Casascius Addict".
Custom titles are also given when theymos does name changes, eg for
BitcoinPenny (formerly CJBianco), who carries their old name as custom title "aka CJBianco".
tl;dr: Everyone can have a custom title, not everyone can set one for themselves.
So how do you request theymos for a custom title or do you need a certain kind of trust in the forum in order to request and obtain one? Also do you need to pay a certain amount depending on the title you want? Because I also don't think that everyone can have that kind of privilege here in the forum.