My oldest son had a medical condition that required shots every 3 days
for the first 2 years.  I always nursed him during the shot.  Basically, he
would pause and open his eyes wide, then nurse furiously for several
minutes, but he never bit me, nor did he have to cry during shots.  We also
nursed during blood tests, they were regular too.  I just did it and didn't
ask permission, usually gave the tech pause, but I just explained that he
wouldn't cry.  Afterwards, the tech was glad not to have had a screaming
baby and didn't object again.  The only thing is to be sure mom can free the
baby's foot easily (or thigh in this case).  With succeeding babies, I
always nursed for shots and I advise all the moms I work with to do so.
Never have heard a complaint in over 20 years.
        Jacie in the Great Southwest