http://www.sutherlandiaopc.com

Yes, you absolutely can apply oleander topically to the cancer and often get great results so long as you are persistent and apply it a few times daily over a period of time.  Iodine painting often works well, as does colloidal silver and baking soda.  The best bet there is to make your own very concentrated oleander extract and make up a topical skin cream as per the directions in this article:

http://www.tbyil.com/oleandersoup.htm

Yes, the problem with oral oleander and dogs was mostly diarrhea and vomiting.  It's even worse with cats.  A minority of humans have digestive problems too, though most usually adapt when they start very small and work their way up.  Some don't though and for those who don't sublingual or rectal is a better option (and I would vote for rectal).

There are no instructions for rectal delivery and it might be a bit tricky with a dog, but mainly just get a rectal syringe and administer it and then figure out a way to hold it in for a few minutes - perhaps by keeping the syringe in and keeping the dog calmed as much as possible.

Also note that inositol/IP6 has had very, very good results with canine and other animal cancers.