Since I found a workaround for this problem, I'll answer my own question:
(Note this is similar to the workaround for the machine room airlock bug discussed in thread
http://www.frictionalgames.com/forum/sho...p?tid=2208)
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Go to ~/Library/Preferences/Frictional Games/Penumbra/Black Plague/ and find a saved game file in one of the subfolders. It should be a saved game from somewhere in the chemical lab level.
Open the save file with a text editor and search for "cavein02". Two lines above should be this line:
<var type="1" name="mbActive" val="true" />
Change "true" to "false"
Then read down about 22 lines below the "cavein02" line till you find these lines:
<class type="cEngineBody_SaveData" name="">
<var type="3" name="mfMass" val="0.000000" />
<var type="1" name="mbActive" val="true" />
<var type="1" name="mbCollideCharacter" val="true" />
Change both "true" values to "false".
Next search the file for "cavein03" and make the same set of changes as you did for "cavein02".
Save the file; start the game and load the save game file you just modified. The blockages in the escape tunnel should not appear allowing you to pass through the tunnel and continue the game normally.
BE SURE to keep a copy of the original save game file in case you mess up when editing it. If you make the wrong changes to the save file, it can crash the game when trying to load the save file. Just go back to the unmodified file if that happens and try again.
I hope this helps anyone who ran into this problem as well. Good luck.
P.S. The site
http://mod.frictionalgames.com/ and forum thread
http://www.frictionalgames.com/forum/sho...p?tid=1176 have useful information for anyone wanting to make modifications to the save files or game.