I'm not educated enough to elaborate other than just the simple basics that Nitrous requires alot more PSI than Co2 and you'd most def have to have your swingarm done and tested to hold such pressure optimum PSI is 900-1000 .. heat on a hot day may become an issue causing your PSI to go well beyond this safe optimal range.. I have to purge my bottle when it reaches around 1100 - 1200 and that's with it only in the sun for an hour while waiting in the staging lanes and it's located underneath the bike in the middle of the swingarm somewhat shaded..
not sure on how a syphon tube would actually work in a swingarm since the Nitrous will travel within that swingarm once it begins to empty out so you won't most likely be able to make use of ALL the nitrous in that swingarm due to the gravitational force of it being thrown around in there.
bottles most often are upside down without syphon tubes or upright or on their sides with syphon tubes going to the most back part of the bottle so when the bike is going forward and the Nitrous is being forced to the back of the bottle then the tube will pick up what is being forced to that location within the bottle....
as far as filling.. I have a 2 lb bottle on the bike and a 20lb bottle I use for refilling.. I keep an extra 2 lb bottle on hand to swap out so that I can then refill the other bottle before it's needed again. Easy and quick enough to do.
as for fillin your swingarm.. you WON"T find any refill station to fill your swingarm. They would loose their license if they did and the liability of something happening is not worth their business. They will only fill stamped bottles. I believe the certification is good for about 5 yrs?? not exactly sure on that.. after that they have to be recertified and get a new date stamp before they will refill it.
also in order for you to do your own refilling.. the bottle doing the transfering has to be at a hotter temperature than the bottle you're refilling.. so you put the empty bottles in a cooler full of ice or your freezer at home... no idea how you will be able to acheive this with a swingarm.
bottles have safety discs that will burst if pressure reaches beyond 3000 PSI (I believe is the rating).. just a gauge for you to have some idea what your swingarm would have to hold safely... I'm sure the bottles must be rated for more than the discs.
also just a lil side note... to have 7 lbs of Nitrous vrs 2 lbs at your disposal at one time???.. not worth the hassel in my opinion.. I can easily swap out my bottle twice (3 bottles) and get the same amount of usage.... but that is just my opinion based on what lil I have learned this summer spraying at the dragstrip
sure someone out here can give you a more technical reason as to why you either can't or shouldn't....