Not quite answering Harsha's question, but I thought that I'd share that Rādhā is mentioned earlier than the 11th c., namely in two places that I can think of:
(a) an Apabhramsha verse by the poet Govinda is cited as an example of the mattavilāsinī-mātrā by Svayambhu, who wrote in the 9th c., in Svayambhūcchandas 4.10.2 (p. 56 in Velankar's edition):
hari suṭṭhu vi āarēṇa
dēi diṭṭhi jahiṁ kahiṁ vi rāhī (rāhihe?)
kō sakkaï saṁvarēvi
ḍaḍḍhanaaṇa nēhẽ̄ palōṭṭaü (ḍaḍḍhanaaṇu?)
“Although he looks at all the gōpīs, Hari is no doubt giving special attention to Rādhā. Who can conceal a glance that has already been sent with affection?”
(b) a Prakrit verse in the Sattasaī (v. 89 in Weber's edition), which has often been considered to be an interpolation just because it is about Rādhā. (The date of the Sattasaī is a big problem, but if anyone wants to argue that this verse is later than the 11th century on textual grounds, go ahead.)
muhamāruēṇa taṁ kaṇha gōraaṁ rāhiāi avaṇentō
ēāṇa vallavīṇaṁ aṇṇāṇa vi gōraaṁ harasi
”Krishna, in blowing the dust away from Rādhā with your breath, you’re taking away the consideration you’ve paid to these other cowherd women.”
There are probably other references if one looks for them (which I haven't done).