You can leave pockets of magma by laying down a layer of magma and then pouring water on it down from above while blocking the water from reaching the tiles you want to leave. Let's say you're casting a 7x7 tower.
Z=0
OOOOOOOOO
O+++++++O
O+++++++O
O+++++++O
O+++++++O
O+++++++O
O+++++++O
O+++++++O
OOOOOOOOO
O = Constructed walls
+ = Constructed/natural floors.
If you fill this with magma and then pour water over it you'll have a 7x7 cast obsidian square. Repeat this above as many times as you want to build the tower up.
Now let's say you want to leave some magma in the tower. All you need to do is block the water you're pouring down from touching the tiles you want to leave. You can do this a few ways, but one of my favorites is by building artificial floors to block the water that will be transformed into natural obsidian floors. Let's say you're 3 levels up now. You still have the artificial walls, but now the floors are natural obsidian. You want to leave one tile of magma in each corner. You start with just the artificial molding walls.
Z=3 step 1
OOOOOOOOO
O+++++++O
O+++++++O
O+++++++O
O+++++++O
O+++++++O
O+++++++O
O+++++++O
OOOOOOOOO
O = Constructed walls
+ = Natural obsidian floors.
Go ahead and cast the whole thing with obsidian.
Z=3 step 2
OOOOOOOOO
OoooooooO
OoooooooO
OoooooooO
OoooooooO
OoooooooO
OoooooooO
OoooooooO
OOOOOOOOO
O = Constructed walls
o = Natural obsidian walls
Dig out most of the level, leaving enough obsidian walls in each corner to support the artificial floor tile we'll build above.
OOOOOOOOO
Ooo...ooO
Oo.....oO
O.......O
O.......O
O.......O
Oo.....oO
Ooo...ooO
OOOOOOOOO
Go up to the next level and it looks like this:
Z=4 step 1
OOOOOOOOO
O++...++O
O+.....+O
O.......O
O.......O
O.......O
O+.....+O
O++...++O
OOOOOOOOO
O = Artificial walls
+ = Natural obsidian floors
Now you can cover the tiles you want to keep with artificial floors.
Z=4 Step 2
OOOOOOOOO
O++...++O
O+*...*+O
O.......O
O.......O
O.......O
O+*...*+O
O++...++O
OOOOOOOOO
O = Artificial walls
+ = Natural obsidian floors
* = Artificial floors
Fill Z=3 with magma again, and then pour water down from above. You'll end up with this:
Z=3 final:
OOOOOOOOO
OoooooooO
Oo7ooo7oO
OoooooooO
OoooooooO
OoooooooO
Oo7ooo7oO
OoooooooO
OOOOOOOOO
This is great, you say, except that there are constructed floors on the level above that you can't engrave. Well, that's true for now. If you want to leave the magma below exposed you could just deconstruct the floor. If you do that be sure to construct it out of a non-magma-safe stone that will just melt when it falls into the magma.
If you
don't want to leave it exposed and want to replace it with a natural obsidian floor the solution is easy. Just cast Z=4 with obsidian, and the artificial floor will be replaced with a natural obsidian floor that can be engraved. Now you have a pocket of magma, and every tile in your tower can be engraved! Note that this same procedure will work for larger pockets of magma in any shape you want to make