Dear bhante Sujato,
Greetings from Bodhinyana! I think your project is a good thing and I hope you are doing well. Here are just some thoughts. Maybe they can be of some help.
metta - ‘Loving kindness’ still sounds artificial to me even after hearing it many times. ‘Kindness’ and ‘goodwill’ are too weak. ‘Love’? I’d say yes! In context it will already have different connotations to people. (like ‘a mind filled with love’ or something)
mudita - I once came across the word ‘compersion’: [1. The feeling of joy one has experiencing another’s joy, such as in witnessing a toddler’s joy and feeling joy in response.] However, I had never heard of this word before, and neither will anybody else. 
‘Rejoicing’ is rather nice, but it is still not really natural. In English we say ‘I am happy for you’, so perhaps ‘happiness for others’? I’m not sure how it well work in context, but at least it instantly gives me a hint of the emotion.
jhana - ‘Illumination’ feels more like insight (aloko udapadi?). Also, ‘the third illumination’ sounds wrong to me. For similar and other reasons I do not really like ‘absorption’ either. As ‘jhayati’ is usually ‘meditation’, perhaps ‘jhana’ can simply be ‘meditative state’? Nice and neutral, adopting existing terms that people understand–as it seems to me the Buddha had done–instead of inventing our own. The context will make it clear that it is not just any state.
bhikkhu - Cool, I like ‘mendicant’ and was already using it in myself! It gives the renunciant feel which actually is inspiring as well. ‘Bhikkhuni’ can be ‘female mendicant’, but usually the pronouns will make it clear anyway:
‘One morning, after dressing herself and taking her bowl and robe, the mendicant Vajira entered Savatthi for alms. When she had … etcetera’
nibbana - I agree. It simply does not mean ‘emancipated’ or ‘freedom’ or ‘unbound’ or whatever. You might as well use ‘cheese sandwich’ for ‘madhu’, just because you don’t like honey. And not translating ‘nibbana’ is missing the point that it is just another term. It deserves no special treatment above peace, liberation, cessation, fading away, ending etc. Actually, it is just a metaphor… So ‘extinguishment’? Yes, please! It’s not upsetting, it’s nice. Or perhaps I’m just cold hearted, but I personally never once got emotional when I saw a flame go out!
A slight problem with ‘extinguishment’ of course, is that ‘to extinguish’ means ‘to put out’ rather than ‘to go out’. Wherever it works, perhaps translate the verbs with ‘to go out’ instead of ‘to be extinguished’. The connection between the two is easily made by people. I might get something like:
‘Immediately after leaving the fourth meditative state, the Lord fully went out. When the Lord fully went out, upon his full extinguishment, the venerable Ananda delivered this verse: …’ dn16
Something like that… still learning! But at least it makes things alive, and it sort of sounds like something somebody might actually say (although ‘attaining nibbana’ would nowadays be more common…)
samadhi - I like the idea behind it, but ‘convergence’ sounds weird. It also reminds me of mathematics… which required a lot of concentration. 
‘Stillness’, I agree, there are already other terms with that meaning. Perhaps is the least of all evils, though? I would definitely pick some translation, not leave it in Pali. ‘Ecstatic stillness’ and such like, are just too artificial to my taste. Its like another cheese sandwich. I also would love some term that has this ‘one-going’ type of idea. ‘Unification’ sounds sort of decent, but also just as cold and dry as convergence. ‘Undividedness’? I don’t know, to be honest. Please let us know how you go and what you settle upon.
Question: Are there any places in the suttas where samadhi is used outside of the context of meditation?
Please keep us updated. I’m learning from it as well and am tinkering on some of my own translations as a practice, using some of your ideas as well. If you’d like, I might post a few of my attempts at some point. Perhaps I have a few decent ideas you can use. Let me know. Sometimes it’s the small things that make a lot of difference, instead of picking certain terms.
It’d be nice to see some of your attempts too, when possible.
With compersion…
Sunyo
he he he hehe
Thanks Bhante & Blake for the info (and all your work). Whatever works is fine with me, I’m just very grateful to have SC!