Here is the key Sanskrit passage.
sa yathā sarvāsām apāṃ samudra ekāyanam |
evaṃ sarveṣāṃ sparśānāṃ tvag ekāyanam |
evaṃ sarveṣāṃ gandhānāṃ nāsike ekāyanam |
evaṃ sarveṣāṃ rasānāṃ jihvaikāyanam |
evaṃ sarveṣāṃ rūpāṇāṃ cakṣur ekāyanam |
evaṃ sarveṣaṃ śabdānāṃ śrotram ekāyanam |
evaṃ sarveṣāṃ saṃkalpānāṃ mana ekāyanam |
evaṃ sarvāsāṃ vidyānāṃ hṛdayam ekāyanam |
evaṃ sarveṣāṃ karmaṇāṃ hastāv ekāyanam |
evaṃ sarveṣām ānandānām upastha ekāyanam |
evaṃ sarveṣāṃ visargāṇāṃ pāyur ekāyanam |
evaṃ sarveṣām adhvanāṃ pādāv ekāyanam |
evaṃ sarveṣāṃ vedānāṃ vāg ekāyanam || BrhUp_2,4.11 ||
Very briefly, the rendering “direct path” was from Nyanamoli, who based it on a single occurrence in the Pali in this sense. In the early Pali texts, this is the only occurrence outside of satipatthana, so he took this to indicate that this was the ordinary language meaning, and hence inferred from that to the meditation meaning.
This is normally a sound approach. However, in a subsequent article, Rupert Gethin drew upon a range of Sanskrit sources, demonstrating that ekāyana has a range of meanings, including the sense “direct”. Thus there is no reason to think that “direct” must apply in satipatthana. Rather, the Pali texts simply happen to have a smaller range of contexts for this term than the Sanskrit.
While Gethin’s article was useful, in my view he didn’t clearly distinguish between the various Sanskrit sources. It’s true that ekāyana has a range of meanings, but it is only used in a philosophically significant sense in one meaning, which is the passage above. This passage is from a section of the Brihadaranyaka that is proven to have deep and complex connections with the Pali texts, so it should be regarded as one of the, if not the, closest of all Brahmanical texts to the Buddha’s context.
That this is, in fact, the relevant context, is proven by the fact that the phrase ekāyana magga is, in the Pali suttas, placed into the mouth of Brahmā at SN 47.18:
Atha kho brahmā sahampati ekaṃsaṃ uttarāsaṅgaṃ karitvā yena bhagavā tenañjaliṃ paṇāmetvā bhagavantaṃ etadavoca: “evametaṃ, bhagavā, evametaṃ, sugata. Ekāyanvāyaṃ, bhante, maggo sattānaṃ visuddhiyā sokaparidevānaṃ samatikkamāya dukkhadomanassānaṃ atthaṅgamāya ñāyassa adhigamāya nibbānassa sacchikiriyāya, yadidaṃ—cattāro satipaṭṭhānā.
I discuss the various texts and contexts at more length in A History of Mindfulness. One of my basic arguments is that MN 10/DN 22 Satipatthana is clearly a late and composite text, and in a case such as this, the shorter and simpler texts of the Samyutta must have been the building blocks from which it was constructed. Hence SN 47.18 is likely the original context in which this term appears, and from here it was later swept up into “the” Satipatthana Sutta. Along the way, the Brahmanical context was lost, and hence the key to understanding.
I regard this as a solved problem. Not only does this yield a very satisfactory sense, it explains why the Buddhist tradition—including both the Pali and the Chinese—seem so confused about this term. They didn’t understand it, because they weren’t the target audience: brahmins were. Brahmins knew what it meant because it’s an important term in the Brihadaranyaka. The Buddhists forgot what the actual meaning of the term was, so in exegesis they proposed a series of possible interpretations.