I have the same problem as you. In the TOC, I remove the underline of the hiperlinks, then I force the <a> tags to display as blocks and finally I add a dotted border bottom to that tags. I do that by adding in the .css file the following code:
Code:
a {
text-decoration: none; /* to eliminate the original underline */
border-bottom: 2px dotted blue; /* a new dotted underline */
display: block; /* so the underline goes from margin left to margin right */
}
In the epub all works fine, but when I build the mobi/kf8 file with Kindlegen, the original underlines are there again
(and the dotted border too). I couldn't find a solution for that issue; I think it's must be a bug of Kindlegen.