If it's only semi-permanent color does it actually wash out after 28 washes? I know that Lucille Ball used to henna over bleached hair but I'm not sure how it affected the actual hair shaft. I got a perm once with henna'ed hair and it was a minor disaster...my hair was breaking off and looked burned. You may want to try a streak before you do your whole head.
Sorry it took so long to answer...I've been without a computer at home for quite some time. Did you ever try this?