I would create an extension where the Alerinfo gets added and then use the local extension:
Add the following lines so you add the header for the phones that you want to get the sip header:
Exten => 1111,1,SipAddHeader(_Alert_Info: Blah)
exten => 1111,2,Dial(SIP/100&SIP101)
Then for your dial statement you will use the Local extension and the sip extension of the phones that you dont want to recieve the alert
exten => 100,1,Dial(SIP/200&Local/1111@Context)
The above line will ring SIP/200 with no extra headers and will ring all the phones in exten 111 with the headers.