You are using an obsolete version of Asterisk (insecure=very hasn’t been recognized for some time now), so it is important to say which version you are using. (Alternatively you never needed to make the peer insecure in the first place.)
I wouldn’t expect Asterisk to allow what you want, but it is easy to try for yourself. Alternatively, look at the definition of the sip_peer/sip_user structures, in your version. Unless they contain a field for the user agent, it will not be configurable on a per peer basis.