that will allow you to use servos without a headache.
Continous rotation may be a requirement - servos dont have that, and if they do they have lost the positioning ability.
I have been considering a new project for a small/simple/cheap lighting system
essentially one Arduino 328 could control 8 stepper motors and 8 lights/ledpanels/mirrors (to be decided)
when I have looked into such stagelighting stuff, the insides are very simple - just like you describe. Apart from the profit stuff, the price for the bought units includes the development to get the mechanics sturdy and reliable and simple interfacing/cascading. (And that is not so easy as one would like to think)
Still it is ALWAYS great fun to do your own
Has anyone else had any experience with writing to stepper motors in a serial fashion
Well, sorry, not me personally no. But I have tried "homegrown" drivers (like you propose, assumes you have unipolar steppers) and ready bought ones, and the latter is easier/faster with just step&dir pin, and microstepping comes included.
I see no obvious reason for your design not to work, as long as your step rate is "low enough". There may be software complications (or nice puzzle/challenges to solve
) for steppers to move smoothly at different rates. I would consider using two 1-16 decoders into the driver Step/Dir instead of a shift register. Pros and Cons.