I took up MA when I had to quit Rugby due to knee injury.
In answer to the original question, I'd say you've got it the wrong way round.
There is little in any MA that is going to benefit you on a Rugby field other than very general things (I'll come back to that).
On the other hand, there is much in Rugby that is good for MA.
What other sport includes Grappling, Striking and Multiple attackers moving at full speed on unreliable terrain, and with opponents of unlimited size and malevolence?
There's grappling, there's clinchwork, there's sprinting for your life, teamwork, peripheral awareness.
You'll get hit hard, without body armour.
You'll try and deal with multiple attackers.
There are no rounds, no mats, no cages.
Once you've fish hooked, kidney punched, testicle squeezed, cleat raked, rib knee'd, head butted, hand offed, nipple bitten and generally battled your way through around 80 minutes of pumped adrenalin and CV work, you get to stand at the bar and down a few ales with the guys who just tried to kill you.
Getting back to MA in Rugby;
Guys/Gals who are into their MMA should go practice against Rugby players.
If you want to practice your 'shoot', then tackling a Rugby player running at you full tilt on a muddy pitch, is the best test you will find. period.
Small joint manipulation is something not many Rugby players know about.
Same goes for pressure points and general nasty MA things, but Rugby players have their own tricks, and MA people could learn a few things in a scrum, ruck or maul.
I'm looking forward to people poo pooing the above, as I have a particularly evil collection of pics from Rugby Matches.