Peckish

First of all, I take it you plan on NEVER washing any of your quilts?? 😳

Second of all, if you use good quality fabrics, threads, and batting, and take the time to sew all seams securely and quilt it properly, your quilt will not "be torn into shreds and pieces" by washing. I'm flummoxed by this assumption and am curious as to why you think this would happen? Some shrinkage is normal and actually desirable to a lot of people, but if you want minimal shrinkage, then plan ahead and pre-shrink all your supplies before assembling the quilt.

I would not use the Frixion (heat-erasable) pens. There have been many, many reports of the ink not coming out, staining or bleaching the fabric even after washing. The manufacturer of these pens does not recommend use on fabrics. There are several products on the market that are designed specifically for marking on quilt tops: air-erasable pens, water-soluble pens, and chalk. A lot of show quilters use the water soluble pens, so that might be your best option.

eta: You can always use a sliver of soap to mark on darker quilts, but then there is that pesky washing thing....