Sorry, I'm with everyone else on this one. A session maibock is a helles.
Boil the crap out of the first runnings from a Helles recipe and add 5 extra hop cones at flame out and call it a "Session Maibock" if you want, but it's still a Helles.
Even it's alternative name "Helles Bock" kind of implies this. I know the literal word-for-word translation is just "Light" "Bock beer" but "Bock" also has a history of being used as a modifier to denote strong; I can't remember the beer author who talked about "bockifying" this and that beer style. From this perspective, Maibock's other name, "Helles Bock" can reasonably mean just "stronger Helles" in which case a "Session Helles Bock" would most certainly just mean Helles.
[Edit] I stand corrected; GREAT reference, dfhar! -From the way the language and bock modifier is normally used, I can certainly see why everyone jumps to the conclusion. -Seems to be a very similar issue to Double IPA just being "an IPA where you double all the ingredients except for water" -a great story and at a distance very roughly accurate-ish, but not really how you formulate a Double IPA recipe (not a good one anyway).
It would seem that a session Maibock is just essentially an Oktoberfest.
Dfar, I'm within you on a more modern Oktoberfest, but a traditional Marzen recipe is already starting darker than most Maibocks and starting from a Marzen grist and ramping it up to bock strength would make it even darker and, IMO, wouldn't resemble most real Maibock examples. (Having said that I like your proposed grist as long as its Vienna malt's 3.5 SRM and not Light Munich's 9-11 SRM at those percentages.)
Sounds like it's occupying the small middle ground between Helles and Vienna with a focus on floral hop flavor
Adam