terrible idea in my book, unless you intend to keep you jacket on at all times. it is kinda like a woman carrying her gun in a purse, which may be set down at time, which means gun is not in her possession. Same thing would go for the jacket if you stop someplace, you take it off and then gun is no longer on your person. I just carry my Glock in a good in the waist band holster and I don't have to worry about it. I carry mine in a MTAC from the holster maker Comp-Tac.
Haven't tried the MTAC, but use a CTAC inside the waist and a pair of their paddle hosters for IDPA shooting, these guys know a thing or three about holster design. I just ordered two more holsters from them last week to add to my collection, and would recommend them highly to anyone in need of a quality holster.
I prefer a 3/4 length jacket, which makes drawing from under a jacket using a belt holster less than ideal. I'm also not fond of carrying anything along my belt line, because the last time I tested the durability of my riding gear I bounced off my hips and back several times. Subsequent bruising was not pleasant. I can only imagine how much worse that would have hurt with even a pocket pistol between me and the ground. The chest might not be any better, but I think a layer of foam behind the firearm could at least lessen the impact.
Carrying in a jacket is really no different than carrying in a purse. If a person decides to carry in this manner, the jacket should never be left unattended at any time, for any reason (if it is "loaded"). That being said, I know a few women who carry every day in their purses, and you would be hard pressed to get it away from them without a struggle. It is simply a matter of understanding the risks and changing your habits to lessen those risks. It goes far beyond simply never letting it out of your sight, but that is a good start.
As for jacket design, I would like to see a design similar to the jackets offered by Sig Tactical. (But make the pockets accessable from outside the main zipper, unlike Sig's version.) A large chest pocket on both sides, velcro lined for unlimited holster options, and a long vertical zipper close to the jacket centerline. The jacket should have 1/4" or 3/8" thick CE type foam between the pockets and the rider to affer at least some protection from impact in the event of a get-off and subsequent impact. A zipper closure would simply look like another vent zipper to the untrained eye, and easier than velcro to open with a gloved hand.
A pair of large concealed outer pockets like these could be used for things like a wallet or cellphone, not just a gun. A phone in the tankbag is useless when you are lying in the ditch a hundred feet from it. I speak from personal experience on that one.
David