this is just a suggestion that i think may help, Can there be a place where we can add our none guild alts, it will help you if a player decides to run with a none guild alt, you will have the reference to hand to credit the players achievement, now this will be a voluntary listing and up to the player to fill it in.
for a start i have 34 other alts on this server all level 70 at different tiers, healers, tanks, dps and ranged, some like this character geared towards PVE, others toward PVP. both sides rep and imps, if a event is being planed with a skill hole you could look this list up and ask the player if they would not mind joining the event on that alt, filling in the hole.
The website already accommodates that. You can add all your characters to it and signup to events with any of them.
Our event leaders don't generally have time to wade through hundreds of characters spread across the 20+ signups some events get. If a particular role needs filling then they ask the people in TS and then they ask in /g.