I was pondering my next build, and decided it would be a large sky scraper with several floors. I tried to build the frame using the BC filler.. but alas it doesn't work at the moment.

I thought maybe I could use some other method to build, and had the idea to cast the walls using lava instead... so several buckets later I have some pretty impressive looking lava drapes

I was thinking I would just keep them as lava for now rather than turn it to cobble. Solves the in-door lighting problem quite neatly! I showed it to Geuis who was concerned because on a different server the admins thought that large lava flows caused server lag.

The lava is "flowing" as in from a source block to some end point below, but the actual route it takes has stabilised and is no longer changing. Is this going to cause a server Lag problem ? I don't want to add to the strain.

any flowing liquid (which means anything that's not a source block of said liquid) will increase server strain and large areas of flowing liquid will increase lag..
I always make sure I don't have any flowing liquid I don't really need..
If you could replace all those blocks with source blocks, it would be fine, but I'm afraid in any other way it's still considered a "flowing liquid" and will increase lag (especially if your walls are as big as you say)
I'm not saying eliminate all of it, but try to reduce as much as you can..

OK Got it.
I dont know of an easy way to place source blocks of lava on such a large scale. I'll do what I was intending originally which is to turn it to stone.

use a floodgate from BC (that fills a cast area or pool etc with fluid put in at the top).
Or
Could use a fountain from minefactory reloaded (could pump in lava and if it's in a tube could shoot lava quite high) just remember the upgrades