I have come across a couple of Cantonese words transliterated in Jyutping which had * in the tone number. For example, on CantoDict, the word
广东话 is transliterated as
gwong2-dung1-waa6*2. What does that asterisk mean? Is it some form of sandhi?
It's not standard Jyutping, but CantoDict uses the asterisk to indicate a changed tone. In your example,
waa6*2, the standard citation tone for 话 is 6, but when pronounced in the word, 广东话, its tone changes to a rising tone, so it is denoted with a
*2. A note at the footer of the definition page indicates this convention:
Also, CantoDict uses a unique "asterisk (*)" convention, to show readings such as
jyu4*2. For more information please see CantoDict Tone Conventions.
Addendum: Since you mentioned sandhi, I just wanted to note that Cantonese changed tones is different from tone sandhi. This previous answer explains in further detail: https://chinese.stackexchange.com/a/9599/166
That's called 连续变调. In Chinese dialects such as Wu(spoken in Zhejiang Province and Jiangsu Province and Shanghai City), Min(spoken in Fujian province and Taiwan) or Cantonese(one member of the Yue Dialects), there exists the alterations of characters' tones when they are used in words. That's called 连续变调.