I've been doing a few BIAB brews with some success, but the only issue I have is I only have a 19L stock pot, so can only do half batches (2.5 gal). I was therefore wondering if anyone had done a small sparge step after the initial mash to increase the volume, and if so, what method have you used?
For my 5 gallon BIAB batches, I just heat 2 gallons of water to 170* or so (in my old extract 5 gallon kettle), tie up my brewbag with a rubberband, place in the ale pail I'm going to use for fermentation, pour heated sparge water over it, teabag it a bit (heh), then dump into the boil kettle after about 10-15 minutes, which I usually start bringing to a boil during the sparge. I've achieved over 90% mash efficiency on mid-gravity beers with this method.
Copying most of my answer from https://homebrew.stackexchange.com/a/12575/3925
I've adapted my technique for BIAB over a few batches. These steps are my version of merging BIAB with all-grain and sparging. I've used this technique with all-grain recipies, and associated water temperatures for mash and sparge, and it turns out great.
I got some buckets from WalMart, drilled holes in the bottom of one, and put the grain bag in the drilled bucket, and then that bucket into a second bucket. This will let it drain without having to hold it up the whole time. Have an extra catch bucket ready for when the first one starts getting full. (Helpful for the next step.)
Not having used the full boil volume in the pot, we need more water. I heat up another few gallons, and begin pouring it over the grain bag in the drilled bucket. The fermenter bucket below is big enough to hold the extra volume you'll be pouring in there. This serves to rinse more of the sugars out of the grain, while topping off the volume. This is my version of sparging.
While you don't get the filtering benefits of a true sparge this way, you can still rinse out the extra sugars, while still being able to capture the runoff from the bag, without having to hold the bag over the brew pot for 20 minutes. Beeersmith gave me a 3 step sparge for an all grain recipe once, so I just used the volumes and temperatures in the recipe, simply poured over the bag in the bucket, wait about 5-10 minutes for the water to filter through the bag each time. It works great, and I can usually hit the target OG for an all grain recipe.
Sparging is done to rinse more of the sugars out of the grist which makes more efficient use of the grains, but that's kind of the trade off with BIAB.
You probably could heat up more water and dunk your bag into it to get more sugar out, but you are still ultimately limited to the size of your boil kettle.