multiple planes (fills) in eagle
I have some connections ... which could benefit from a plane ... especially all connections to power

Quite unlikely.  Ground planes are certainly useful, but most instances of power planes are due religious convictions rather than physics.  What makes you think you need a power plane?  In some cases of high current, it may be a suitable way to deliver that current without much of a voltage drop, but then you still have to consider the high current connection points to the plane.

Most likely you don't need a power plane at all.  Instead make sure that each use of power is well bypassed to the ground plane.  That gives you low impedance at high frequencies.  Fat enough traces give you low resistance at DC.

However, to answer your question, the RANK setting in Eagle is exactly for the purpose you seem to be describing.  With the right relative rank settings, the power polygon will dominate in its small region within the larger ground plane.

From the Eagle help page for the POLYGON command:
Defines how polygons are subtracted from each other. Polygons with a lower 'rank' appear "first" and thus get subtracted from polygons with a higher 'rank'.
Valid ranks are 1..6. Polygons with the same rank are checked against each other by the Design Rule Check. The rank parameter only has a meaning for polygons in signal layers (1..16) drawn in a board and will be ignored for any other polygons. The default is 1.

This really is quite clear.