Safe 0–300 VDC variable supply
Collected Information from the Comments:


The lack of a transformer in the second circuit is prohibitively unsafe, so I'm going with the first schematic.
1:1 transformers like the one used in the first circuit are, in fact, relatively cheap—e.g., one candidate for $14 and another for $18.
The input voltage for the first circuit is actually 220 VAC—an oversight on my part. However, I can link the output windings of the above transformers in series to achieve my desired output voltage.
However, the voltage and current maximums achieved by the circuit are, in fact, just "ball-parks". Since the circuit deals above the benchmark for lethal current, a variable maximum is far from ideal.
The lack of a fuse on the primary side would render the first circuit illegal in many places. 
A 300V 100mA supply is potentially lethal, so don't touch both terminals at the same time. Circuits with dangerously high output voltages shouldn't be built by people who don't know what they're doing (or have only a loose grasp).