This filter is based on sidki's Fix <base> Target (http://"http://asp.flaaten.dk/pforum/topic.asp?TOPIC_ID=784"), but for a very specific use:
In Opera, sidepanel links always open in the sidepanel window (unless target=foo is used in the sidepanel pages). Using a quick rewrite of your code, I now found a way to get all sidepanels to open in new windows 
[Blocklists]List.SidePanel = "..ListsSide Panels.txt"
[Patterns]
Name = "Opera Sidepanels open in new window"
Active = TRUE
Multi = TRUE
URL = "$TYPE(htm)&$LST(SidePanel)"
Limit = 256
Match = "(<bases*)9"
"("
"(target=$AV(((_|)top|self|parent)3)7&$SET(1=9target="_blank"7))"
"|"
"((target=$AV(3))87&$SET(1=987))"
"|"
"(>&$SET(3=-empty-)&$SET(1=9 target="_blank">))"
")"
"|"
"((</head>)2&$SET(1=
<base target="_blank">
))"
Replace = "12$STOP()"
All sidepanels used would have to be listed in the SidePanel blocklist.
My list:
# Proxomitron4 URL killfile: $LST(SidePanel)
#
# by Jor
#
#
# Blocklist for use with the 'Sidepanels open in new window' filter. This file
# lists all sites currently used as Opera sidepanels.
#
custom.opera.com/~stephan/nu.nl.cgi
europe.cnn.com/desktop/content.html
my.opera.com/customize/panels/?rdf=
(The last entry is an RDF/RSS interpreter I use with many RDF or RSS feeds, including Slashdot)
Edited by - Jor on 15 Aug 2002 02:11:12
A second filter for Opera sidepanels, taking care of the problem these cannot be refreshed:
[Patterns]
Name = "Sidepanels: Add [Refresh]"
Active = TRUE
URL = "$TYPE(htm)&$LST(SidePanel)"
Limit = 256
Match = "<body*1>2"
Replace = "<body1><a href="u" target="_self">[REFRESH]</a><br />"
"
2"
Edited by - Jor on 15 Aug 2002 02:45:20