In reading this thread I am struck by some misperceptions that may be confusing to the OP. Before addressing these it is important to realize that there is a difference in performance between strings using 100% polyester and some of the newer strings which are blended poly or poly-based. I intend to focus my comments on the blended polys because these are more modern and more suited a wider variety of player.
Myth #1 - RPM last will lose 10# in 15 minutes. I am in full agreement that RPM Blast is not one of the better performing blended polys on the market, however, when installed at lower tensions, and without overstretching, it will definitely NOT lose 10# in 15 minutes. However, if it is installed like a synthetic, (without a full appreciation/understanding of how poly-based strings react to tensions greater than the low 50's and fast pulling speeds), then the results reported are not surprising. It is extremely important for there to be an effort to preserve the elasticity when installing these strings. Unfortunately this is a message that is slow to gain traction with many stringers.
Myth #2 - Stringing at lower tensions without overstretching makes no impact on the performance of the string. Absolutely false. Polys have very limited elasticity and this elasticity must be preserved during the stringing process. When it is preserved blended polys will hold tension remarkably well and provide solid performance for an extended period of time. (Actual life will depend on the string as well as how precisely it was installed.)
Myth #3 - You can not play with polys for more than 20 hours/1 - 2 weeks is the maximum life for poly. In terms of pure poly, this may very well be accurate, but in terms of the more modern poly based offerings, it is not. Poly based strings can give very good performance, (and not promote arm pain), for more than 20 hours when installed at lower tensions with the elasticity preserved.
I have a bias against the stiff Babolat frames and believe those with some additional flex are better candidates for low tension stringing with a full poly-based string setup. For 100 sqin frames with open patterns I find that tensions in the low to mid 40's tend to be a good starting point. For denser patterns the upper 30's is a good starting point.
I believe the OP will be able to get exactly what he is seeking with a more flexible frame using a good poly based string installed in the suggested tension ranges above. He will easily be able to get 20+ hours of play and should not experience any arm discomfort and the poly based strings will help him find easier access to spin. The same results *may* even be found with his Babolat frame using lower tensions.
To the OP: Good luck with the Head frame and resist the urge to string polys in the same tension range as synthetics. They are simply not designed for those tensions.