Can we use 230V AC 16amp extension lead for 100V AC 16amp output?
Can I use them?

As long as you don't exceed the rated voltage or current limits the cables themselves will be fine.

What could be the issues?

A UK 16A campsite lead will be fitted with blue industrial plugs and sockets, in a style variously known as Commando, BS4343, CEE17, IEC 309 or EN/IEC 60309-2. This style is available in a range of connectors for various voltage ratings and are colour-coded (blue = 240V) and keyed to avoid plugging equipment with different voltage ratings together.
If you make up an adapter between your 100V inverter, possibly fitted with some type of Hubbell connector, and your 240V IEC 60309-2 leads then at some point in the future someone, possibly yourself, will connect your 100V supply to a load that needs 240V to work safely or connect a 240V supply to a 110V appliance.
You can get yellow IEC 60309-2 connectors intended for 110V use. (Note in the UK 110V industrial appliances are usually supplied with 55-(0)-55V rather than 0-110V supplies.)