Generally, the ICA protocol has improved and been extended with HDX, since the MetaFrame days. You should be able to run over 56k, but it probably won't be a great experience.
There are lots of tricks and configuration tweaks you can consider, to improve low-bandwidth high-latency performance.
These options include:
- Disabling drive, port, audio and printer mappings (unless really needed)
- Restrict printer bandwidth if printers are needed
- Use Flash redirection
- Disable themes, window animations etc. in Windows
- Experiment with local text echo and other SpeedScreen settings
- Use aggressive image compression, maybe progressive too
This can largely be configured by policies, so you can provide a barebone, but faster, experience to modem users, and a rich experience to broadband connected users.
To hit the sweet spot takes some trial-and-error, so being able to test is vital to get it "right". Consider a small PC running WANem, pfSense or M0n0Wall, that can act as a bridge with configurable bandwidth, latency and packet loss.