I don't think accessing the tumor would be a problem or too difficult in most cases. There's a whole subspecialty dedicated to getting needles in different parts of the body using image guidance (interventional radiology). If you can see it on an imaging modality (CT, ultrasound, MRI, etc...), you can probably get a needle there. This is how many biopsies are done to make a diagnosis, by getting a needle into a tumor using CT, ultrasound, etc... and taking tissue samples. The needles required to inject such small doses of drugs would be even smaller and safer than many biopsy needles. Small needles can often even be guided through structures like the lungs, bowel and aorta safely under image guidance.

The other issues you mention about translation to humans are true - it is still early/preliminary, but definitely exciting!