Ron,
They do make flat faced idler pulleys that are made specifically for tensioning or minor back-bending of the belt to achive minimum wrap around the pulleys (similar to the idler found on the serpentine drives of your trucks - the truck idler is spring loaded so it automatically adjusts for belt stretch and wear, increasing belt life). Your (or borrowed) idea would also work well. A take-up mechanism makes the initial tensioning go very easy and potentially more accurate and makes any subsequent checks and adjustments also easy. When there is not an easy way to tension v-belts, most will not take the trouble to check and possibly re-tighten when needed.
I suppose most don't see the value but then who knows how much different the results would be. Would your belts last longer, would your pump last longer, would your motor bearings and seals last longer? I definitely know that on production equipment where all maintenance was tracked on computer and when belts were tensioned properly, and rechecked and adjusted as required, everything in connection with the drives lasted longer.
Believe me I went around and around with several plant folks before they finally realized the benefits. Like I said, they now use very sophisticated set-ups and electronic measuring devices to lengthen part and equipment life. Obviously it would be an overkill for a P/W but the basic principle remains the same - tension the belt(s) properly.
Regards,