Is constant current achievable with a dc/dc converter?
The idea behind the capacitive discharge circuit is that you can charge it as slowly as you like, and then release it all quickly when needed. The current capability of the supply doesn't affect how the starters are ignited, it just affects how long it takes to recharge before the next launch. 

The way this works is, your power supply can provide 15 mA for several seconds (or even minutes) to charge up the capacitors. The capacitors charge according to the formula

\$\dfrac{\mathrm{d}V}{\mathrm{d}t} = I/C\$

When charged, the stored energy in the capacitor is 

\$ E = \dfrac{1}{2}CV^2\$

This energy is what you will use to heat up your igniters.

Then you disconnect the supply and close a switch to connect the capacitor to the igniter, and the capacitors can release its energy quickly into the igniter. The power supply isn't involved at all during the time when the igniters are being powered.

One thing to watch out for is, if the power supply is vastly underpowered, it will be spending a long time at the beginning of the charging cycle driving a much lower voltage than it is designed for (while the capacitor voltage is ramping up from 0 to near the inal value). Depending how the supply is designed, it might not last long when used this way.