I finally received the list of virtual directories from the development team.
This is what I ultimately used...
Create a new string Data Group containing the legacy URI
ltm data-group legacy_uri
records {
/custa/
/custb/
/custc/
...
iRule
when HTTP_REQUEST
if { [class match [string tolower [HTTP::uri]] starts_with legacy_uri]} {
set uri_leg [class match -name -- [string tolower [HTTP::uri]] starts_with legacy_uri]
set uri_new "/app.web/APPvirtualDirectory/"
HTTP::uri [string map [list $uri_leg $uri_new] [string tolower [HTTP::uri]]]
}
}