As long as one of your wireless computers is always on, then it will work. If it is set to sleep when inactive then it would not be a good candidate.
First check that the computer you want to monitor responds to ping requests (some antivirus/firewall software prevents this).
First turn off the other wireless computer so only the one you want the router to watch is turned on. Go to DD-WRT Status Sys-Info tab and scroll to the bottom. It should show only one wireless client now and you'll need to write down its MAC address and its IP address. Write down Host name if it shows one (ignore if just shows an asterisk).
Now turn the other wireless computer back on and get a command prompt (Start, Run, cmd.exe) and type "ping 192.168.x.x" where the x.x completes the IP address of the computer you want the router to monitor. If it responds with "Request timed out" then the machine you want to monitor does not respond to ping/echo requests so you'll have to resolve that with your antivirus/firewall software before you can continue. If it responds with "Reply from 192.168...." then you can continue.
Go to DD-WRT Services tab and under DHCP Server section, click the Add button to create a new blank line. Type in the MAC address (with colons), host name if it showed one or leave blank if not, IP address (192.168.x.x), and use 1440 for client lease time (24 hours). Click "apply settings".
Reboot the computer you want to monitor and then check again the DD-WRT Status Sys-Info tab to make sure it came up with the same IP address you wrote down before (it should always get the same IP address now when it connects to your router).
Now go to DD-WRT Admin Keep-Alive tab and in WDS/Connection Watchdog section, click Enable, use 300 for seconds, and put in the IP address of the wireless computer to monitor (192.168.x.x). Click Apply Settings.
Now you can test it by turning off the computer to monitor and wait 5 minutes for router to notice and reboot. You can either notice the lights on the front of it cycling through reboot sequence, or check DD-WRT from the other computer - it shows "up" time since last reboot in the upper right corner.
Of course if you turn off or sleep that computer that the router watches, the router will keep rebooting every 5 minutes until you turn that computer back on. I have no idea if that is "bad" for your router if you turn your computer off nightly and let the router reboot itself all night long.
You could change the 300 seconds to less if you want it to notice wireless dropout sooner, but I wouldn't go less than 120 seconds or so to avoid an endless reboot cycle if it can't find that computer. You need at least enough time to get back into DD-WRT to turn off the Connection Watchdog if something goes wrong later on. Five minutes seemed like a reasonable time window to me.
I have heard some rumors in the DD-WRT forum that DHCP static leases "don't work" but it seems to be working for me with Buffalo DD-WRT 14998.
Good luck.