okay, dunno if this will help you, but I have a bit of a story (yay story time!!!)
anyway last week my old Textiles teacher (she was a cow, but shes gone now! haha) was being the usual pain in the butt (cleaning up when the lesson was still going rather than paying attention to us) and she got a whole load of dye or fabric paint on a light coloured shirt. Anyhow she wiped the majority off, and I suggested using a hairdryer to dry it off until she can go and get it cleaned. As you probably know, wet clothes arent fun. especially if you have a big wet patch on your right breast!
anyhow she replied "no, otherwise that will seal it in. If you dye or get colouring into fabric, heat seals it"
So maybe she was useful for something! I dunno!
Also, as for the hairspray suggestion, I've only ever seen that being done in my art classes to make sure things like charcoal and chalk stay put on paper and wont smudge. im not 100% if it applies to ink and fabric, you could always try a sample.
Dry heat does help to set a dye or stain into clothes, so if you put your jeans into the dryer, the sharpie will likely last a bit longer. That being said, however, you are still going to wind up re-coloring your pants every so often. There are very few dyes that don't fade over time, and sharpie is not made to be a fabric dye in any case. The best thing you could do to keep the color would be to find a dye made specifically for fabric in a similar shade - a fabric marker would be best for the result you are describing.
As for keeping the back half of your pants from turning pink, I don't really know what to tell you. It's very likely that the sharpie will bleed and get onto the rest of your clothing - in fact, I would highly recommend not putting anything else in the wash with the pants for a while, or wash by hand. COLD water helps keep color from bleeding out in the wash, but be prepared for it to bleed some anyway. .
I'm not sure how it applies to Sharpie, but hairspray actually removes normal ink. I had a pen break in my pocket, and to get the ink to wash out my mom told me to spray it with hairspray before washing (it worked).
I atteneded a free class at the local libary - it was tye-dying a T-shirt with sharpie. We colored designs in sharpie, and then we doused the shirt in Rubbing Alcohol (sp?)
The colors spread and created this awesome tye-dye effect. I think that if you made a small bath of Rubbing Alcohol in your bath tub (just enough to cover your jeans completely.) The colors would spread, but if you washed them by themselves, it should stick.
I would reccomend dyeing them with something like RIT instead of using sharpie, just protect the rest of the pants and use a spray-bottle to get the dye where you want it. But if you want to use sharpie there are a couple of options- you could try soaking the pants in vinegar/hot water, I know that helps set a lot of dyes, it might set the ink into the fabric. You could also buy Textile Medium, which is clear, and paint it over, but I'm pretty sure that makes the fabric kind of hard, because it's an acrylic product.
Hope that helps!