Meanwell are a good make so you are off to a good start in selecting them.
I always rate heater SMPS at twice the required heater current otherwise they often won't even power up into cold heaters and ones with hiccup mode current limit work better. On the other hand, do not over rate it too much else it may end up operating in hiccup mode because the load is very light and I know for certain this does stick crap on the heaters which definitely does find its way into the audio path.
Having said that, I now use Meanwell 12V SMPS for the heaters of all my mixers with no additional filtering. For 700mA your idea of a linear reg is fine but you probably will not need the extra filter in between.
A dc/dc converter for HT is something I am looking into right now. I too have been thinking of generating it from the heater supply. However, winding this up to 250V or so is not so easy with a simple circuit. Someone has already done this in a 500 series module use a MAX something chip (can't rememeber the number off hand) - I will see if I can find the details. Anyway, I have designed a small PCB with this max chip to try it out. I expect the boards in the post any day now. However, I contacted MAX about this question and they put me in touch with an application engineer. Turns out the big problem is getting the ripple down to an acceptable level so it looks some additional filtering will be required there also. There are many, many other chips to choose from ( although I am limiting myself to DIP types) so I may just not yet have stumbled across the best one.
Cheers
Ian