I am happy to report that a pre-treatment of argan and coconut oils seems to have prevented damage from conventional hair dye, and actually improved dye uptake.

I've decided to take a break from henna, and opted to use a semi-permanent dye to color the new hair growth, and touch up fading color on the length. After washing with a mild shampoo, I let my hair dry for a bit, then applied the oils from roots to the ends; enough to saturate but not feel greasy. I left this in overnight, and dyed my hair in the morning. I used conditioner to rinse out the dye, which was not quite effective enough to remove all the oil, but my hair felt fine afterward. Normally it would take a few days for my hair to recover from from the dye process, and require lots of pampering. I CO washed again the next day, which removed the remaining oil residue. There was no noticeable dye runoff after this second wash, which has never happened before. Usually my hair would bleed color for several washes.

Thanks so much to ktani for starting this thread, and everyone who posted about their experiences. I'm still leery of using conventional dyes on a regular basis, but it is good to know that I can use them occasionally without the risk of serious damage.