Let's discuss Iris Havengard

Okay, what's the best way to go about coding something like that?

There's a line in the SwdTech command (at C2/185B) that overrides its normal function if the Bushido index is 2 (the Retort condition).  I think we can build on that concept.  Basically, we use the index of each menu option to branch to alternate code that changes the attack element, queues an attack, then runs a RNG check, and then finally sets a spell and queues a second cast from it (if the RNG result falls at or below the roll).
By forking based on index, we have the option to set different probabilities for each element (making stronger spell calls like Doom or Pearl less likely), but the command will take up more free space.

Using that approach wouldn't override weapon spell triggers (since the first attack would always be handled by the Fight command), setting up the possibility of Iris casting two spells in a row, one from her weapon and another from her MBlade.  That's not necessarily a problem (Relm casts spell after spell after spell while fingerpainting in my other hack), but it's worth noting.

FYI, this approach would build off Catone's work, meaning each menu option would list the spell Iris might cast rather than "Flametongue" or the like.

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One more question... since I work in direct HEX instead of ASM, any code I put together for this won't be floating code (that is, it's not going to be easy to move it about).  For that reason I need to know... is the Iris Edition to be built off a vanilla experience, off the C.V. Reynolds Bugfix compilation, off PowerPanda's 3 Scenarios, or some other founding hack?