Forgive me if this question could be answered by me digging out my copy of Chainmail, but does the #HD = #actions rule allow for casting multiple spells in one round?
That's a good question - That would allow for casting multiple spells in one round!
The slower rate of HD increase and the limited spell slots should balance it out, though I could see to it leading to the equivalent of a tactical nuke (good ol' flammable web + fireball) in one round on a large enemy.
Something that comes to mind is that in most OD&D retroclones, spells can be interrupted by missiles. If one side loses initiative and gets interrupted before they cast multiple spells, I would personally rule that it interrupts all the spells they were going to cast that round, making it potentially riskier to do so.