I think the IR blocking of the DERF would be a good precaution, even for white light observation.
The issue is simply one of coefficient of thermal expansion, and the fluoro-glasses, including FPL 51, 52, and 53 are all significantly higher than "standard" glasses. For the cemented element lens, this likely caused the failure. Any mechanical impingement could possibly also result in failure.
Then there's the issue of changing optical figure when used for solar, which I have real-world experience with in the SolarScope filters. These use fused silica etlaon plates, which have a far less coefficient of thermal expansion -- and they are subject to significant fluctuation with temperature gradients caused by solar, and destroy the above arguments about lack of sunlight falling on the glass affecting them... especially as they are preceded by an RG630 ERF with IR blocking (the Coronado patent for the central spacer for maintaining a uniform etalon gap was definately needed in the larger filters.)
Bottom line, I will not use my irreplaceable AP130 EDT objective for unprotected solar observation -- its not worth it from a structural failure point of view, and would not provide as stable optical performance as an "ordinary" objective in any event. It is confined to H alpha use with a DERF, or white light use using Baader film, and not a Herschel wedge.