It is possible to use Inno Setup Unpacker to extract the files from the setup and then manually copy the files over the installation. That's what I did to update Impulse version, it
should work with Steam, version, too, long as EU3 is copied outside of Steam's directory like beta patches.
Inno Setup Unpacker
Don't try this if you are not familiar with the command line.
1. Install EU3 + Divine Wind if needed. Copy the fresh installation somewhere else, I'll use D:\EU351 as example. This keeps your original installation untouched so you can easily install the 5.1 patch to it once your distributor releases it.
2. Download the 5.1 installer and save it somewhere you remember, I'll use D:\eu3patch here.
3. Download the latest version of Inno Setup Unpacker and extract it somewhere you remember, I'll use D:\innoup here.
4. Start Command Prompt. Type
D:\innoup\innounp -x -dD:\eu3patch D:\eu3patch\eu3_v5.1.exe. That should give long list of file names and the fles should be extracted to D:\eu3patch\{app}.
5. Copy files from D:\eu3patch\{app} to your copy of EU3 Divine Wind installation.
6. Done, clear the map cache as usual and enjoy.
edit: Oops, forgot to add link to the Inno Setup Unpacker.