This is just a list of the unused icons, i'm not going to bother with a full list right now.

Buildings:

• White square - 44, 47,48, 49, 50, 51

Units:

• Bird - 5
• "94" - 93
• similar to Town Patrol icon - 97
• Dolphin - 99
• Elite Eagle Warrior tech icon - 114
• Stormy Dog - 133

Techs:

• Hand cannon? - 24
• Cannon with Cannon Balls - 26
• Axe Head - 33
• Longboat painting - 41
• Trebuchet - 56
• Alternative scorpion - 68
• Scroll and Ink - 77
• Shadow of a boat? - 95
• Book and cancle - 108
• Book and necklace - 109
• Sign - 116
• Cancel 'X' - 117

Well, you could make the building a unit with no movement speed. I think it won't be able to set gathering points though.

You can also export the BMP file of the unit you want and add it the the SLP file where the building icons are.