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If you don't care about efficiency of the overall system, then a zener to limit the voltage will work.  You are right in that a series resistor is not needed since the source is power-limited.

One problem with a zener is finding one guaranteed to limit the voltage to the 5.5 V maximum while not getting in the way under normal operation.  If the normal operating voltage is 4.5 V as you mention, then you only have 1 V of room for the knee, tolerance, and to find the right voltage in the first place.  That might be tough.

Keep the power dissipation in mind.  The absolute maximum is (6 V)(150 mA) = 900 mW.  That's unrealistic, but you don't really know what fraction of that the zener will end up dissipating.  You should make sure the zener can handle the power even with the switcher disconnected.  I'd use a 2 W zener.  Fortunately that's not hard to find.

<blockquote>Or am I going about this wrong and is another type of over-voltage protection preferable?</blockquote>

My first reaction is to find the right MPPT chip for the solar panel you have.  I haven't looked so don't know what's available, but something that can handle up to 8 V in would work nicely without kludges like artificially clamping the panel voltage.