A simple workaround is to call the remote commands through a wrapper that writes the PID to some file or to STDERR:

#!/usr/bin/perl
print STDERR "PID: $$\n";
exec @ARGV;
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For example:

$ ssh my.host.com tellpid echo 'hello world'
PID: 15079
hello world
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Also, check this ticket on the OpenSSH bug tracking system: Bug 1424 - Cannot signal a process over a channel (rfc 4254, section 6.9). That feature, scheduled for inclusion in the next release of OpenSSH, would allow to send signals to the remote processes.