Having an iPad continuously powered for extended periods is not an issue - and will cause no harm - as this is common practice in POS (Point of Sale) and Kiosk applications.
iPad will optimally manage battery charging - and may automatically change its charging strategy by reducing the maximum battery charge level to about 80% of maximum capacity. This strategy is intended to extend the life of the battery by reducing chemical ageing that occurs when the battery is continuously maintained 100% charge. When a “normal” charge/discharge pattern is resumed, charge to full capacity is restored.
Other than by manually disconnecting the Power Adapter from the iPad, there is no method by which you can limit the maximum charge-level of an iPad.
More information about iPad’s battery management can be found here:
About iPad and iPhone Charge Management Feature – Apple Support