I can create a target from the command line with something like:

omp --config-file=.ompconf --xml '
<create_target>
<name>254 IP Subnet</name>
<hosts>192.168.0.0/24</hosts>
</create_target>'

But let's say I wanted to exclude two IP's from that subnet because they belong to printers. How would I do that?

You need to list the hosts individually, or use multiple masks, as can be calculated using ipcalc.

I was thinking I might have to just make a list of all 250+ IP's. I'm not sure how multiple masks would work, though. If I use smaller subnets, what about the extra network and broadcast addresses?

For instance, let's say I wanted to skip 192.168.0.66:

192.168.0.0/26
192.168.0.65
192.168.0.67
192.168.0.68/30
192.168.0.72/29
192.168.0.80/28
192.168.0.96/27
192.168.0.128/25

But then this assumes extra network addresses of:

192.168.0.68
192.168.0.72
192.168.0.80
192.168.0.96
192.168.0.128

And extra broadcast addresses of:

192.168.0.63
192.168.0.71
192.168.0.79
192.168.0.95
192.168.0.127

So won't the scanner skip all those extra network and broadcast addresses? Or can I get around that by listing those all separate at the end? If so, then my final list should look like this?

192.168.0.0/26
192.168.0.65
192.168.0.67
192.168.0.68/30
192.168.0.72/29
192.168.0.80/28
192.168.0.96/27
192.168.0.128/25
192.168.0.68
192.168.0.72
192.168.0.80
192.168.0.96
192.168.0.128
192.168.0.63
192.168.0.71
192.168.0.79
192.168.0.95
192.168.0.127

I think I would block the printer IPs with iptables rulesJust make a rule for each printer.