For IVA there is no actual requirement to have a rollbar. Although welding isn't obviously as good as a single piece of tube there shouldn't be any great issue with your hoop - I presume it is CDS and not welded or even worse scaffold pole?
It is worth noting that if you mount yout seatbelts to the hoop structure then it becomes part of the 'seatbelt' system and as such must be deemed suffciently strong by the IVA man to resist any impact. AshG had an issue on his test where the inspector deemed that the single bolt at the bottom of the diagonals would not be strong enough and had to fit 2 bolts. I've done the same on mine to avoid any potential problems - have a look at photo 170 in my gallery here
http://www.facebook.com/media/set/?set=a.245243755324.181913.601005324&l=a9831a9319& type=1
Out of interest what are the chnages you have made to the rear structure for ?