Paul, I was resident in Pattaya/Jomtien for many years and active in the bike scene there. Obviously, as you have already surmised, Pattaya is full on and breeds it's own scams and worries for anyone wanting anything from a local. The only place for big bikes I would hire from is the Muslim family at the beach end of Pattayaland Soi 2.
The brother is Amin, President of Burapha MC and a very well known, well liked and influential biker in Thailand. However, this is not to say they may not have some unscrupulous folk working for them..... it's a big firm and you may possibly deal with someone on the day who treats you like just another Pattaya 2 week WESTie.
If you don't know him, I would take the time to nip up to Amin's bike business on Soi Batman (Sukhumvit road) to say hi, show an interest and then enquire about hiring a bike...... he would probably make a phone call to his sister, who is in charge of the hire operation, and put you on the right track. I have no problem with you dropping my name if you want: Martin from Phaiyanaak MC and Ace Cafe in Jomtien (now closed).