Nothing official was made for the IIsi and I don't believe it has physical accommodations to move the feet. A lot less effort went in to making the undersides of the IIsi, LC, and most other later desktop Macs pretty enough to use the machine in a convertible way, which seemed to be a pretty specific goal of the IIcx/IIci/Q700.
From a physical standpoint, there's no real reason you couldn't just position it vertically and use it that way. I don't know off hand what the airflow in the IIsi looks like, so you may want to put it on particularly big rubber feet.
As a sidenote, Gateway favored stands kind of like these in the 2000s for their P4-based small desktop systems, mainly because the systems were too curvy to just run vertically.
For a while, I was thinking about the best way to apply rubber feet or a plastic sheath to the bottom and one side of the Outrigger (7200+, g3 desktop) case to run them vertically, but the plastic is not very structural on that model, so if Apple were to have done that on their own, the sheath would itself need structure and would probably have held the system by the bottom.
In a lot of other "vertical stand" situations, it's just for extra stability or to create a certain look if you're worried.
As an interesting modern sidetone, the latest Dell OptiPlex 3040, 5040, 7040, and now 5050 and 7050 systems (skylake and kabylake business desktops) have feet on two surfaces, so you can run them horizontally or vertically. The systems are proportioned closer to a shorter IIci, probably so that you don't waste an awkward amount of desk space putting one underneath an LCD display.
Another modern side-note, Apple is said to officially condone running the modern Mac mini vertically. I can't find the KB off hand, but the answer was essentially if you can make it not fall over it won't hurt anything.
