You can test Shore A on irregular objects, but the problem that arises is non-repeatable results.  I've used a hand held durometer, and to measure the hardness I needed a flat surface so the durometer wouldn't "roll" on the sample and give erroneous readings.

If the readings you need are more for your information, an irregular shape should give a close reading.  If you're using the info to report, you should make a flat witness panel from which to get your readings.