Large HOA assessment for my rental unit, where to plug in the number?

No, you can not just add it to "Maintenance".  In general, maintenance is something that you would do to maintain the rental unit at a basic level.

These massive renovations are capital improvements and will be added to your basis of your rental unit.

 A capital improvement is something that actually adds to the value of the home or increases its usefulness.  Some examples of capital improvements are:

• Remodels and room additions (including decks and porches)
• New or upgraded landscaping, irrigation, sprinkler system
• Hardscape such as pavement, block or retaining wall, patio
• Fencing
• Swimming pool, spa
• Storm windows, doors
• New roof
• Central vacuum or security system
• Upgraded wiring, plumbing, ductwork
• Central heating, AC, humidifier
• New furnace, water heater
• Filtration, soft-water, or septic system
• Built-in appliances
• New flooring or wall-to-wall carpeting
• Upgraded insulation
• Satellite dish