I'm pretty sure Sasktel is the same as Bell and Telus and runs an IS-96 network, not a GSM network. The GSM coverage in Sask is Rogers' alone. I suspect the lack of Rogers HSPA may be due to bandwidth availability and spectrum ownership within the province.
Whether or not Rogers roams on Sasktel's own HSPA network is something that I do not know. I suspect they don't considering the coverage in the province.