How do I get bc(1) to print the leading zero?

You can also resort to awk to format:

 echo "0.1 + 0.1" | bc | awk '{printf "%f", $0}'

or with awk itself doing the math:

 echo "0.1 0.1" | awk '{printf "%f", $1 + $2}'

This might work for you:

echo "x=0.1 + 0.1; if(x<1) print 0; x" | bc

After a quick look at the source (see bc_out_num(), line 1461), I don't see an obvious way to make the leading 0 get printed if the integer portion is 0. Unless I missed something, this behaviour is not dependent on a parameter which can be changed using command-line flag.

Short answer: no, I don't think there's a way to make bc print numbers the way you want.

I don't see anything wrong with using sed if you still want to use bc. The following doesn't look that ghastly, IMHO:

[me@home]$ echo "0.1 + 0.1" | bc | sed 's/^\./0./'
0.2

If you really want to avoid sed, both eljunior's and choroba's suggestions are pretty neat, but they require value-dependent tweaking to avoid trailing zeros. That may or may not be an issue for you.