Place your pointer on the .BGL file and after RIGHT clicking, choose "Open With"...Then choose "WordPad" as the application to open it with.
The result will be a mess of unintelligible rubbish...However place your pointer up at the beginning of it and click once. The go Edit > Find...in the find window type ".BMP" without the quotes and click the Find Next button.
The highlight will go to the name of the first texture used and if you keep pressing the Find Next button it will tell you the name of every texture used by that .BGL file.
I wish I had known this a long while ago...would have saved me many binary searches.
Try it now