The way I would do it is:
You start out with an asymmetrical playfield routine and you set up the bits
to represent the coarse health (4 pixel chunks) and then you can fill in the
gaps (1-3 pixels if necessary) with a sprite colored the same as the
playfield. You'll have to do some math and/or have some tables to convert a
number into a combination of playfield registers, horizontal sprite position
and sprite data. I would align the sprite on the PF boundary and adjust the
shape to use between 0 and 3 bits rather than actually overlaying it, so
that when the health is near zero you won't be stuck with a sprite that's
too fat to represent 1-3.
You should be able to get two health meters on the same scanline that way.
You can position the missiles to draw a line representing the maximum
health.
Here is a crude diagram:
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx BLUE
XXXXXXXXXXXXXP0 M0 XXXXXXXXXXXXXP1 M1 BLACK (BG) RED (FG)
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx BLUE
The blue scanline above and below help "frame" the space between the end of
the health meter and the missile lines.
As the health deteriorates you can adjust the color registers, maybe
shifting from green to yellow to red. There should be enough time to have
the two health bars be independent colors, rewriting both the necessary
playfield register and the COLUBK register.
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