It could be considered a variant of "body writing" but occurs in mediums besides body writing, and is fairly specific in it's form. And finding examples of it...took a fair bit of time.

Essentially, any instance in which the possible depth of penetration is visually represented by lines, said lines can show up on the abdomen, be indicative of anal or vaginal penetration (possibly oral as well), or show up on a penis, or even be shown on a screen near the characters in question.

Simply put: Visual Representation of Possible Penetration Depths in a scaled manner.

Example 1: http://ehentaihip.com/s/455492e596/2666399-29

Example 2: http://ehentaihip.com/s/9d8331f077/2498210-8

Example 3: http://ehentaihip.com/s/5ef8c753a2/2686597-511

Example 4: http://ehentaihip.com/s/17f1907fcc/1389781-1

Example 5: http://ehentaihip.com/s/ea8266140d/2355494-3

As shown, it is a rather specific kink in art that is not limited to just body writing, thus hunting for content with it is a tad daunting.

Thus I have put forward the tag, which is already in use in some image hosting sites.

Depth_Grading

I feel it would be a apt and descriptive tag that would be of use.