Sadly, my home machine can connect only by modem. Does BOINC know to dial the modem when it's time to trickle results, or is there a way to have it call a script?

In theory it does, but apparently it is notoriously bad at it. It probably is better to monitor Bionc using one of several scripts and programs and dial up yourself when it needs to return a trickle or result, telling it to update the project when you do (sorry - cant remember the 'update' command - buried somewhere on the boinc website)

<a href="http://www.boincstats.com/"><img src="http://www.boincstats.com/stats/banner.php?cpid=7cbf470a82da52fef2efcb6e2d35844c" border="0"></a>

PS: remember with CPDN at least you can wait for several trickles before returning them :)
PPS: You have to change your server-side settings to reflect that you are using dail-up