I am not sure that the prostate friendly saddle is so important in your case - research indicates that professional cyclists are no more at risk of PCa than other men, they just acquire a higher 'normal' PSA count. Men that are recovering from surgery are advised to avoid cycling for a few months to give their nerve bundles a chance to recover (not applicable to you I guess) and men that have just done a big ride may have a slightly higher PSA reading which should be borne in mind before panicking.
Important to check what your onco thinks but it seems to me that as long as you don't do a big ride in the couple of days before your regular PSA tests, you will be fine?