We really no longer recommend oleander in any form for cats - their systems just cannot handle it from virturally every report we have had. I have no personal or anecdotal experience with bloodroot and cats, but see from a Google search that it has been used - perhaps diluted quite a bit as it is pretty intense stuff. Here is one discussion thread I found:
Colloidal silver has worked very well in combination with inositol/IP6 in many different animals - and it often results in renewed vigor. You might also consider painting the tumor with iodine and baking soda (alternating back and forth) and another item to consider is diluted DMSO. As far as I can tell, ioding, baking soda and DMSO have all been used medicinally for cats.