There's no guarantee it will be read by a sub, but having the appropriate response spelled out in detail is your best hope.
I work as a substitute for our school district and I have never had a teacher leave in the plans which child is on an IEP. If a child is pulled out for services the sped/ssn/etc just show up and say they are taking so and so for whatever it is they are receiving.
The most I've ever seen is "please allow 'John' as much time as he needs" or "positive reinforcement works really well for 'Sarah'" or "'Eric' will need repeated reminders to follow along with the rest of the class or to stay on task".
And then at some schools if the students have any life threatening medical conditions or allergies they are on a master list (usually middle schools or high schools). The majority of the elementary schools are peanut and pet free schools so I don't think that is as much of an issue.
So you will need to perhaps stress that the IEP goals or accommodations will be communicated to the sub in the sub plans. A lot of it will depend on the sub, there are many good subs out there, but there are also a fair number of burnt out or cranky subs and there may be no way around that unless you can get your teacher to find a sub(s) that works well with the class in a positive way that she can request for absences.
Of course I can only speak for my district and places I have worked. This may not be true or possible for every area.
ETA: The only time I've seen an IEP list of students is when I subbed for a special ed teacher so it was to be expected I knew who was on an IEP and for what.
