Is it possible to use the following Irule syntac with TCL in a policy ?

HI Lemaire_Frédéri , conditions in policies don't accept Tcl expressions, only actions. If we could land on a single expression to accomplish this it might be possible, but anything requiring anything more than a cached value lookup in a policy sends me to an iRule outright by preference...your mileage may vary. To your block of code, you can simplify that to not set any variables at all (not as readable), and if you're always going to rewrite if it "isn't" lowercase, then you don't really need the condition and can just rewrite always, right? Anway...IF that's the case for this particular virtual server, THEN, you might yet be able to use a policy with this:

rewrite_first_path_segment {
    actions {
        0 {
            http-uri
            replace
            value "tcl:[string map [list [getfield [HTTP::uri] / 2] [string tolower [getfield [HTTP::uri] / 2]]] [HTTP::uri]]"
        }
    }
    ordinal 5
}

 Again, I'd go the iRule route here, but you can investigate from here with the policy.  A couple other solution options I was noodling on for fun:

# option 1 - close to your solution
set pathobj [getfield [HTTP::uri] "/" 2]
if { ![string equal $pathobj [string tolower $pathobj]] } {
    HTTP::uri [string map [list $pathobj [string tolower $pathobj]] [HTTP::uri]]
}

# option 2 - regex rewrite 
if { [regexp {^(/[^/]+)(/.*)?$} [HTTP::uri] -> pathobj remaining_uri] } {
    HTTP::uri [join [list [string tolower $pathobj] $remaining_uri] "/"]
}