The signings were not witnessed by any member of the SMM Campaign but by the management of the artists; see
www.petertatchell.net for full details.
When you are dealing with people who are prepared to lie (either to you or their fans, to get out of a tight spot) then not sure what realistic safeguards can be attached to such agreements.
I am not sure about pre-emptive action such as sworn affadavits. In awarding performance licences, authorities should be aware of artists' past records [no pun intended]. Being a genuine signatory to the RCA should help authorities judge whether the artist should be allowed to perform. Being a subject of the SMM campaign and not being a signatory suggests that the artists is likely to incite homophobic violence. Authorities should take this into account.
I'm not sure about people being arrested for what they might do in the future. That way madness lies. I think those artists who are under suspicion should be monitored and if in the UK they choose to 'sing' a song inciting homophobic violence then they should be arrested. Current legislation going through parliament suggests an up to 7 year sentence. I hope this gives some artists pause for thought.
I think it is important that the SMM campaign works with the police to ensure that they put resources into monitoring concerts.
For some reason Peter Tatchell is against the extension of incitement laws to include a specific homophobic aggravation as it already does for race and religion/belief. Evangelical Christians and some Muslims are organising to lobby parliament against the law change - the voice of the SMM Campaign would be important in responding to this threat.