Session Persistent based on the HTTP Header

It can. You cannot use the BIG-IP priority group activation method for this, because it has a different meaning than what you describe above, but you can define two pools, one for each "priority group". You would then select the "priority group" based on the header field. You would set the Virtual Server LB method to "Ratio" and define the pool member ratios in each pool. It would look something like this (untested!):

when HTTP_REQUEST {
    if { [HTTP::header exists "iSession"] && [persist lookup uie [HTTP::header "iSession"]] } {
        persist uie [HTTP::header "iSession"]
    }
    else {
        switch [HTTP::header "iPGroup"] {
            "One" {
                pool pool-One
            }
            
            "Two" {
                pool pool-Two
            }
        }
        
        if { [HTTP::header exists "iSession"] } {
            persist uie [HTTP::header "iSession"]
        }
    }
}

In this configuration, you should also define a default pool for the Virtual Server, which would be used if iPGroup is not set or is not a value in the set ["One", "Two"].