Deut 7:9 is not an infinitive, it is a participle. This is
true also in the Samaritan reading tradition (on the
basis of Abu Ishaq Ibrahim's grammar book).
Nevertheless, the point is well taken. While I believe
the Samaritan reading tradition provides evidence that
l- was sonorant and without a vowel in Biblical times,
including in the case of the infinitive as can be seen
in such examples as lc)t [elciyyat] vs cognate Tiberian
Hebrew [lace:t/lå:ce:t], and that the infinitive construct
had a two consonant cluster in the beginning of the
word (as can be seen from your example above and the
cognate Tiberian Hebrew li$mo:r), there is no reason to
generalize this and conclude that a triple consonant
cluster was still maintained in Biblical times. Thus, I
retract my statement earlier and suggest that in
pre-exilic times, the infinitive construct was li$mvr /
$mvr and not l$mvr / $mvr as stated previously.