how to publish wordpress subsite from root / - URI Masking

So if I understand your question, your app is accessible with the /wordpress URI prefix, but you want it externally accessible as simple "/". In order to make this work you have to do a few things:

1. You have to make sure that all response content (HTTP headers and payload) with this URI are mapped from /wordpress to /

2. You have to rewrite the URI prefix from / to /wordpress[HTTP::uri] on each incoming request.

So let's take a look at what that iRule might look like:

when HTTP_REQUEST {
     don't let the server compress responses
    HTTP::header remove Accept-Encoding

     disable STREAM for requests
    STREAM::disable

     rewrite URI if it doesn't start with /test
    if { not ( [string tolower [HTTP::uri]] starts_with "/wordpress" ) } {
        HTTP::uri "/wordpress[HTTP::uri]"
    } 
}
when HTTP_RESPONSE {
     catch and replace redirect location headers
    if { [HTTP::header exists Location] } {
        HTTP::header replace Location [string map -nocase {"/wordpress" "/"} [HTTP::header Location]]
    }

     enable STREAM to replace payload URI content
    STREAM::expression {@/wordpress@@}
    STREAM::enable
}

This is a simple example, so you may have to tweak a little to get it just right, but hopefully it makes sense. You can also accomplish most/all of this with URI-translation rewrite profile if you're running a newer version of BIG-IP.