One exception to the "hired gun" approach. If you are travelling a bit of distance for 1 of your kids to play in a tournament and you are dragging another kid with you- May as well try to get them on the field at the same tournament.Guesting or loaning for inter-club teams is common and can give players valuable experience.
Guesting for another club is when things can get a bit dicey if it's beyond the school, neighborhood, or friends connections.
Playing down guesting well don't think that's a good policy, sandbagging to get some $5 metals are normally just for the parents or the thirsty club(s).
If a player wants to guest or gets a opportunity they will let parents know, no reason to "advertise" a players out that I know of. Coaches communication and they should be the ones giving suggestions or recommendations on additional play.