Following this question, I notified my player that I was thinking of changing the way that her combo would work. Before I could even tell her how that would work, she started digging into the books again for some new, more "conventional" ways to raise her spellcasting limit.
She asked me if I would allow the Complete Mage from D&D 3.5, and I (stupid me) said yes.
Then I looked at her Character Sheet. She managed to bring her Int up to 24 (in a legal way), thus she has an extra 7th level spell slot.
I immediately realized what she's planning - she will put a Heightened Lightning Bolt (7th level) on that slot and use the reserve feat to use a 7d6 "at will" Lightning Bolt.
First, Heighten Spell reads:
All effects dependent on spell level (such as saving throw DCs and ability to penetrate a lesser globe of invulnerability) are calculated according to the heightened level.
And then, Storm Bolt reads:
As long as you have an electricity spell of 3rd level or higher available to cast, you can fire a 20-foot line of electricity as a standard action. This bolt deals 1d6 points of electricity damage per level of the highest-level electricity spell you have available to cast. A successful Reflex save halves damage.
How will the Reserve Feat interact with this? I suppose it will use the effective spell level of the Heightened LB (7th), but there might be nuances that I'm missing.
I'm feeling a little uneasy about letting a wizard have an at-will 7d6 short lightning bolt, but again I don't want to punish players for creative use of the system.
Do these two feats interact in the way she is thinking they do?