I am not able to provide any references, but kindly allow me to offer
some opinions.
The concept of grace is vague in Buddhism comparing to all other
religions. Even in Christianity, grace is only given "in exchange"
for faith. A buddha may be an essential element for Dharma to be
transmitted to others. But a Buddha is not a Hindu avatar or a Jewish
messiah. And it is possible for a person, under the right conditions,
to attain nibbana on his own. This we call a paccekabuddha.
I would not call the Buddha approaching Angulima a gift or favor,
although I am not hesitant to use christian evangelistic words that
Angulima was "ripe to pluck". :-)
Your exploration of the concept of grace in Buddhist literature is an
interesting and significant one. Dr Masefield's work has received
high regards throughout buddhist circles. It is highly possible that
elements of grace do exist in Buddhism, you may regard Buddha's Pure
Land (in the Pure Land school of Mahayana Buddhism) as an example.
However, because in the Asian buddhist traditions, people approach
religion with a different mindset, hence we don't see many works
discussing grace in Buddhism. I am sure your investigation will be an
eye-opener for us, providing us another window to see the magnificent
nature of the Buddha-dhamma.