Several years ago in the Gulf of Alaska with a moderate gale approaching we decided to rig the trysail -- not so much because we would need it, but to try it out in windy conditions. (Prior to this time I had hoisted the trysail at the dock / anchor several times, but had not done it underway with the wind/seas getting up.)
On the recommendation of a crew member (very experienced guy, 100,000 sea miles, circumnavigator, etc.) we rigged the clew of the trysail to the outhaul of the mainsail. We immediately discovered that the sail was not cut to be rigged thus -- although the foot was tight, we couldnít get the boom end down far enough to get the leach tight. The result was we had a big belly in the sail, couldnít point very well and we were heeling too much (especially when we came off the wind a bit.)
We then re-rigged everything so the clew was sheeted to the leeward quarter with a big snatch block (acting as the turning block) and hence to the primary sheet winch. With it setup in this fashion the sail was very flat, the boat pointed higher, and heeling was reduced. I should add that itís a good idea to work out how youíll tack with the trysíl as well. On our boat the mainsheet, boom and topping lift complicate tacking the trysíl. (Perhaps this is the reason that some people suggest securing the end of the boom to the deck -- it clears the area where the clew of the trysíl needs to pass when tacking.)
My guess is that you can get a sailmaker to cut a trysail that can be rigged to the boom, but most storm tryís are cut to be sheeted directly to the deck.
Such an arrangement also letís you adjust the height of the set with a line running from the tack to the deck at the base of the mast.
IMO, the whole deal (getting the main out of the way, the trysíl set, tackable, trysíl down and out of the way and main re-set when things calm down) is a lot easier if the trysíl is both set on itís own track and is rigged to be sheeted independently from the boom/main. In a perfect world, the trysíl should probably have itís own haulyard as well -- it took us about an hour to figure out how to lead the main haulyard through the lazyjacks to get the trysíl set properly. (Another reason why you should practice the set and stow of the trysíl before you need it.