How to dissolve steel without affecting aluminium alloy
The part of the steel insert that is stuck to the aluminum is not accessible to water.  Let's look at linear coefficients of thermal expansion,

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Aluminum is 22.2 ppm/K. Steels are smaller, the largest being 304 austentic at 17.3 ppm.  Ambient temp is around 24 C, a 400 F kitchen oven is around 204 C.  That should not be hot enough to lose temper in the alloys.  If the aluminum hole is an inch in diameter, worst case increasing the temp 180 K times 4.9 ppm/K net gives you 882 ppm or 22 microns of slack.  The steel insert pushes out.