I want to use a BLE tinyduino module with a custom design antenna. It's not to improve typical bluetooth performance, but for a research project.
How easy would it be to replace the antenna on a BLE shield with an external one?

I looked at the datasheet and it appears

BLE112 with integrated chip antenna and software version 1.0

The antenna is an integrated ceramic  on-board type, I'm not sure it supports an external antenna. (Ken correct me if I'm wrong)

Transmit power: +3 dBm
Receiver sensitivity: -93 dBm
Typical line-of-sight range: 30 - 150 meters
Antenna  Type: Integrated ceramic chip

Perhaps Ken will make a v2 or BLE+ shield that will allow external antennas.

You can find more information here: https://www.bluegiga.com/en-US/products/bluetooth-4.0-modules/ble112-bluetooth--smart-module/