Some reel-to-reel recorders do not have DC-driven motors and as such aren't easy to make into something with variable speed, hence variable delay times when set up to do echo. Others do, but you might not want to mod them.
Place a musical-sounding digital delay (Lexicon LXP-5 and others) at the input and route/mix the return/feedback into its input. Viola, variable-delay tape echo =character=, but the feedback will sound a bit different and you won't be able to do analog delay time modulations. Digital distortion also of course factors in so a limiter at the input might help.
Please to experiment.