Email the promoter of the show i would suggest, whoever that is for that show. Not sure how much influence MH's management company have on appointing local support acts, so think promoter is your best bet
When Motorhead headlines, they often have 2 bands opening up for them. Usually the band that plays before Motorhead is fairly well known. The lineup for at least part of the 1982 tour was a group called Fist opening the show. Afterward Krokus came on (they were getting somewhat popular at the time) and then Motorhead came out afterward.
In 2011 when they headlined they had Valient Thorr, and Clutch opening many of the shows.
Very best wishes for that one. I sent him a porody of Ace of Spades as for years in interviews he has said he was bored of doing it, he may well do the parody for the crack to see if anyone notices! Really hope you get the gig!
If Lemmy knew I describe MOTORHEAD as a nice band he would be spitting plectrums !