Hi OP, in my humble opinion I'd tell your client the only way to have a safe link from his site to yours is with a genuine link. Trying to hide it in the ways you mention in your original post could have negative effects on both of your sites, as mentioned they worked a few years ago but are frowned upon now. I'd say you have three 100% safe options really.
1). Make the link be it anchor text or url and blend it in to the bottom of the page beside or in between other small text so it doesn't stand out, in the footer text perhaps.
2). Go for the 'Website designed by yoursite.com', I understand this isn't what he wants but this type of link is overlooked or ignored by 99% of visitors that are only interested in what the page is about, this type of link is obviously also the best for possible future work for you and I would say the safest too for both of you. If there was ever a safe combination of text or anchor text or combination of the 2 it would be 'Website designed by http://www.yoursite.com'.
3). Create an image or use an existing logo / banner and link that to your site. It can be as small as you can get away with without looking like its so small its only purpose is for a link ( so not just a pixel lol ).
These are what I would consider to be safe, white hat methods and as far as I'm aware totally untouchable in the penalty rat race. All the best whatever method you choose.