You don't need a beater per se. I have a '68 Gitane Super Corsa set up fixed and LOVE it (the extra-long campy dropouts are really keen). However, it had no braze-ons and I can convert it back in a heart-beat. You could do the same with any 70's vintage Puegeot PX10 or otherwize which did not have the cable routing brase ons. Frankly, OLDER french bikes (pre 1980) and early 80's Japanese upper end Univegas, Myatas, and Fujis (with decent Tange tupes) are by far the best conversions. Your likely going to have to hack on that 531Pro to get it right, and with th shrt dropouts and light tubes, it won't last like the others I've listed.
Just my opinion. I firmly believe it's your bike and you can do what you want, but it just doesn;t make sense to me.