http://www.mail-archive.com/wix-users@lists.sourceforge.net/msg05097.html gives an solution to republish the properties that were changed in the custom action immediately after it.
Here is an example of how I have gotten it to work in my code:
<?xml version="1.0" encoding="utf-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
<Fragment>
<Binary Id="ConnectionStringExtension.dll" SourceFile="$(var.ConnectionStringExtension.TargetDir)$(var.ConnectionStringExtension.TargetName).CA.dll" />
<CustomAction Id="MyCustomAction" BinaryKey="ConnectionStringExtension.dll" DllEntry="VerifyConnectionString" Execute="immediate" />
<UI>
<Dialog Id="ConnectionStringDlg" Width="370" Height="270" Title="Database Settings - [ProductName]" NoMinimize="yes">
<Control Id="ConnectionStringLabel" Type="Text" X="45" Y="73" Width="100" Height="15" TabSkip="no" Text="&Connection String:" />
<Control Id="ConnectionStringEdit" Type="Edit" X="45" Y="95" Width="220" Height="15" Property="CONNECTION_STRING" Text="{200}" />
<Control Id="Back" Type="PushButton" X="180" Y="243" Width="56" Height="17" Text="&Back">
</Control>
<Control Id="Next" Type="PushButton" X="236" Y="243" Width="56" Height="17" Default="yes" Text="&Next">
<Condition Action="enable"><![CDATA[CONNECTION_STRING <> "" AND CONNECTION_STRING_VALID = "1"]]></Condition>
<Condition Action="disable"><![CDATA[CONNECTION_STRING = "" OR CONNECTION_STRING_VALID = "0"]]></Condition>
</Control>
<Control Id="Cancel" Type="PushButton" X="304" Y="243" Width="56" Height="17" Cancel="yes" Text="Cancel">
<Publish Event="SpawnDialog" Value="CancelDlg">1</Publish>
</Control>
<Control Id="BannerBitmap" Type="Bitmap" X="0" Y="0" Width="370" Height="44" TabSkip="no" Text="WixUI_Bmp_Banner" />
<Control Id="Description" Type="Text" X="25" Y="23" Width="280" Height="15" Transparent="yes" NoPrefix="yes">
<Text>Please enter database configuration</Text>
</Control>
<Control Id="BottomLine" Type="Line" X="0" Y="234" Width="370" Height="0" />
<Control Id="Title" Type="Text" X="15" Y="6" Width="200" Height="15" Transparent="yes" NoPrefix="yes">
<Text>{\WixUI_Font_Title}Database Settings</Text>
</Control>
<Control Id="BannerLine" Type="Line" X="0" Y="44" Width="370" Height="0" />
<Control Id="VerifyButton" Type="PushButton" Width="56" Height="16" X="45" Y="118" Text="Verify">
<Publish Event="DoAction" Value="MyCustomAction">1</Publish>
<Publish Property="TEMP_VERIFIED" Value="[CONNECTION_STRING_VALID]">1</Publish>
<Publish Property="CONNECTION_STRING_VALID" Value="[TEMP_VERIFIED]" />
</Control>
</Dialog>
</UI>
</Fragment>
</Wix>
The CustomAction sets the value of CONNECTION_STRING_VALID to either 1 or 0 depending if it is valid or not and I have defined elsewhere that by default its value is 0
<Property Id="CONNECTION_STRING_VALID" Value="0" />
Now when I click my verify button if the string is valid the next button is enabled