Install Mac OS X Tiger (PPC) to External HDD with an Intel Mac
You may try your luck with Pacifist. This application will take just the installer packages and extract them to where you want them as a destination.

However, the application falls short in a few areas for a really complete installation, as some things were not designed in: pre- and post-install scripts, blessing (making it bootable), ownership and permission will likely be off.

For these problems there is a nice writeup available at How to install a very small version of OS X on an external drive (2007):


  STEP 1: Format your drive (HFS+, journaled)
  STEP 2: Enable owners    

 sudo /usr/sbin/vsdbutil -a /Volumes/CRUZER

  
  STEP 3: Install OS X
     Insert disc 1 of your OS X install media (you can close the Finder window that pops up; you don't need it), then open Pacifist and click the "Open Apple Install Packages" button at the bottom. A pop-up window will open that should have an option like, "OS Install - Mac OS X and standard applications (Mac OS X Install Disc 1)":
  
  Selecting an install package to extract from
  
  Highlight it and hit OK. Pacifist will load the relevant files, which will take a little while.Eventually, it will spit out disc 1 and ask for disc 2.
  
  Expand, "Contents of EssentialSystemSoftware", then expand, "EssentialSystemSoftwareGroup" and select both "BaseSystem" and "Essentials":
  
  Selecting the packages to extract
  
  With those two selected, click on File -> Install Files to Other Disk..., select your external drive in the pop-up window and hit OK. In the next pop-up, leave "Use Administrator Privileges" checked and hit "Install", then enter your password to start the process.
  
  During the extraction/copying process, you might see either (or both) of the following warnings:
  
  A file already exists at the following location...
  
  In response to this, check "Don't ask again for this installation", and click, "Leave alone".
  
  The application bundle [path to bundle] already exists...
  
  For this one, check the same box and click "Replace".
  
  Once this process completes you can open up your drive in the Finder and you should be able to see the system folders and files (along with anything else that was already there if you didn't format the partition in step 1):
  
  STEP 4: Copying additional files from your existing installation
  
  Open a Finder window and navigate to /System/Library/CoreServices on
  your main hard drive. Open a second Finder window to the same place,
  but on your external drive. Now copy the file SetupAssistant from the
  former to the latter. (You'll need to enter your password to confirm.)
  
  Similarly, navigate to /Library/Receipts (note: NOT
  /System/Library/Receipts!) on both drives and copy BaseSystem.pkg and
  Essentials.pkg from the former to the latter.
  
  STEP 5: Make the drive bootable
  
  Open Terminal and enter:
  
  sudo bless --verbose --folder "/Volumes/CRUZER/System/Library/CoreServices" --bootinfo --bootefi
  
  (Note the double dashes for each argument in this command! Also, if
  you're using a non-Intel Mac then you don't need the --bootefi flag.)
  
  After being prompted for your password, you should see a verbose
  output for this command, which should contain a number of indications
  of success, e.g. "boot.efi created successfully".
  
  STEP 6: Fix permissions
  
  In Disk Utility, select your external partition and click "Repair
  Permissions". This may take a little while depending on the type of
  drive you're using.
  
  STEP 7: Boot!
  
  Restart your Mac, holding down the option key until you see the list
  of devices that can be booted. If all went well, you should see
  something like this: