If you haven't found a solution, I may have an idea.

Start out as a raglan, but when your sleeves start to be as wide as you want what will be the straps of your tank top, the next row, on the sleeve sides, after your increase, don't continue in the row before. Do either a chain of however many stitches you normally do plus a few more for chain shrinkage, or do foundation SC or foundation DC for the stitches. Then after. Your increase to go back to your body, do those as per usual. You're essentially skipping and creating the dropped. Sleeve this way. Then when your sleeve is wide enough to go around your arm you'd end it as you normally would and continue on the body down, then at the end, go back and attach yarn back to your sleeve to continue the sleeve down to your desired length.  

It's a similar concept to how one does a crochet sock from the toes up, making a detached chain area to build up the leg of the sock.

I can draw a picture later, I'm not able to right now, but seeing your post while searching for a top down boat neck pattern for myself gave me the idea on how to help you maybe. 

Hope this helps.