We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z ... 7-9 selected to have intense optical line emission by Roberts-Borsani et al. Previous follow-up has revealed Ly... in two of the four galaxies. Our new MOSFIRE observations confirm that Ly... is present in the entire sample. We detect Ly... emission in the galaxy COS-zs7-1, confirming its redshift as z sub( Ly...) = 7.154, and we detect Ly... in EGS-zs8-2 at z sub( Ly...) = 7.477, verifying an earlier tentative detection. The ubiquity of Ly... in this sample is puzzling given that the IGM is expected to be significantly neutral over 7 < z < 9. To investigate this result in more detail, we have initiated a campaign to target UV metal lines in the four Ly... emitters as a probe of both the ionizing field and the Ly... velocity offset at early times. Here we present the detection of C iii emission in the z = 7.73 galaxy EGS-zs8-1, requiring an intense radiation field and moderately low metallicity. We argue that the radiation field is likely to affect the local environment, increasing the transmission of Ly... through the galaxy. Moreover, the centroid of C...iii indicates that Ly... is redshifted by 340 km s super( -1). This velocity offset is larger than that seen in less luminous systems, providing an explanation for the transmission of Ly... emission through the IGM. Since the transmission is further enhanced by the likelihood that such systems are also situated in large ionized bubbles, the visibility of Ly... at z > 7 is expected to be strongly luminosity-dependent, with transmission accelerated in systems with intense star formation. (ProQuest: ... denotes formulae/symbols omitted.)