SO... Just so I get things straight...
That RT8472 could run 2 x 2.5v 350mA Cree XP-C LEDs from a 6v source without issues?
My Current BOM (Feel free to criticize it)
6 x Cree Amber XP-C LEDs (XPCAMB-L1-R250-00401)
3 x Richtek RT8472
Total estimated lumen output at 350mA/LED is 330lm
(Passive components to suit)
Also, how would I calculate total current draw? I realize it'll be bugger all but it would be useful for correct fusing for the circuit
The RT8472 is a great LED driver that I've used many tens of thousands of with excellent success in automotive applications, and it is incredibly cheap! But I don't think it would be well suited to your particular application - it is a buck (step down) driver, with a 5V to 30V input range, so you would be running really close to it's cut-out voltage and it won't be all that efficient, and you would only be able to drive one, maybe two LED's with it.
Instead, I would probably go with the Texas Intruments TPS61165. With just 3 capacitors, an inductor, a diode and a resistor, you're good to go - it will accept 3V to 18V input and it is a boost regulator, so it will step the voltage up, allowing you to run pretty much as many LED's as you want. And if you ran, say, 10 LED's in a string, you will get into the 90-95% efficiency range so the driver circuit will run nice and cool. And not only is the driver cheap, TI will send you some for free

and their datasheets are awesome (IMO) and they basically give you a parts list and a PCB layout. You can run the LED's at lower current (like 100mA or even a bit less) for more efficiency as well as more even light distribution within your housing. Then you could just put the driver circuitry on the back of the same board, fill it with copper pours, do some testing to see what you can get away with for max drive current and you're good to go.
The other nice thing about this setup is that LED current is set with a resistor so you can use a precision 10-turn pot if you like, or use a trimpot in conjunction with a normal resistor so you can adjust LED current precisely in small increments, and then test it in the housing while you let it blink on your bench for several minutes at a time and see what temperature it reaches.
Keep in mind it's the LED junction temperature that matters - on the MLE's it's probably 150C but you won't want to get anywhere close to that - but even staying below 50C board temp, I bet you get massively more brightness than a 3W incandescent bulb.