Note that in the RAW for RM2, spell prep is a 90% activity. (People sometimes don't realize that in the RAW, prep takes more activity than actual casting).
But yes, you could spend 2 rounds prepping to work up the energy for the spell, and then you can use the 'Opportunity Action' (RM2 Arms Law, p. 8, section 3.2 'Opportunity Action') to hold it indefinitely; while holding an action in this way, the only other action you can take is movement at 25% activity. (Essentially, holding an 'Opportunity Action' is a 75% activity).
This would be different in RMU, where holding an opportunity action just takes Concentration (essentially it is just a 50% activity).