If you look at the ZTR Grail rims, you'll notice that it rated for a lower pSI the bigger the tyres;
Max Pressure with 28mm Tire: 100psi
Max Pressure with 32mm Tire: 45psi
The ZTR Iron Cross max pressure is;
Max Pressure with 32mm Tire: 39psi
Unless you're one of those people who decided to run the max tyres pressure said on the sidewall, there's little to no reason why you can't ride an MTB or Cyclocross rims.
If you look at the ZTR Grail rims, you'll notice that it rated for a lower pSI the bigger the tyres;
Max Pressure with 28mm Tire: 100psi
Max Pressure with 32mm Tire: 45psi
The ZTR Iron Cross max pressure is;
Max Pressure with 32mm Tire: 39psi
Unless you're one of those people who decided to run the max tyres pressure said on the sidewall, there's little to no reason why you can't ride an MTB or Cyclocross rims.