DC-DC Up-Conversion without Switching Electronics
A switching power supply is the conventional way.  LEDs shining on lots of photocells in series would produce a higher voltage, but would also be horribly inefficient.  You have 1 W available, so you would need 6% efficiency to get your 60 mW out.  I haven't run the numbers, but that sounds considerably optimistic to me.  You also say low heat generation is desirable, which means you need good efficiency.  LEDs to photocells isn't going to meet that spec most likely (I say "likely" only because you haven't provide a real spec).

You are also confusing requirements with implementation methods.  Apparently what you really want is 25 V or so with very little noise.  Don't assume you need a particular method to get that.  You also haven't provided any spec.  "Very low" is no spec at all, so really any method is acceptable.

I would start with a normal boost switcher, filter that, then run it thru a linear regulator.  Two stages of ferrite chip inductors in series with ceramic cap to ground in front of the linear regulator should get rid of the frequencies the regulator can't handle.  Put a good cap on the output of the regulator too, and you should have a nice quiet supply.