If I have 35+ VLANs coming back to 1 or 2 3750's I can't use HSRP on all of these (according to the 3750 docs only 32 VLANs can be running HSRP) however what about a 2811 ISR? Those VLANs would come in as sub-interfaces on a single trunk interface and the only limit I can see in the documentation is 256 stand-by groups.
Does this mean the maximums are 32 HSRP VLANs on 3750 vs 256 on a 2811?
I think you have a misunderstanding about how using a single HSRP group would work. Perhaps a small example would help:
interface vlan 10
ip address 10.10.10.2 255.255.255.0
standby 1 ip 10.10.10.1
interface vlan 20
ip address 10.20.20.2 255.255.255.0
standby 1 ip 10.20.20.1
interface vlan 30
ip address 10.30.30.2 255.255.255.0
standby 1 ip 10.30.30.1
So you have multiple VLANs and they all use HSRP group 1. But each interface has its own virtual IP address. So this approach should work for you on your switch.
[note] while many of us tend to configure HSRP using a different group on each interface - and may think it more logical and easier to understand, it is not a requirement that each interface use a separate HSRP group.
On a L3 switch you can assign multiple vlans to the same HSRP group so you don't face a limitation as such. You could if you wanted have all your vlans in just one group.