The Furlex20s datasheet http://www.seldenmast.com/search.cfm?downloadmanual=7142685&filename=595%2D496%2DE%2Epdf indicates 500kg max working load. While it doesnt quote it, breaking loads are typically somewhere around 50% higher for most equipment.
The Harken 434 is rated at 227kg and the high load 435 is rated at 431kg http://pdf.nauticexpo.com/pdf/harken/harken-catalogue-2011/21575-23956-_147.html so I would suggested the published Fulex working load figures are somewhat generous to say the least ! To my mind ‘working’ implies you can still turn it !
While checking the Harken site they are now offering a hoistable swivel system which suggest its possible to tension the luff separately from the forestay – perhaps this is the answer to our prayers !
… er hum except its ludicrously expensive:
harken 464 hoistable swivel £190.59 purple marine
harken 479 4mm tack adaptor tang 108.18 purple marine