I heard recently that Harbard was actually Odin. The supposition was in his reaction after tricking Thor he displayed the same patterns as Odin. Similarly I heard that Saga is purported to be Frigg in disguise by some experts.
The identification of Harbard (grey-beard) with Odin is a very well-established one among modern scholars. It is based on the fact that both in the Poetic Edda (Grímnismál 47) and in the Prose Edda (Gylfaginning 20, quoting Grímnismál), Harbard appears in a list of names of Odin.
In general, modern translators of the Poetic Edda like Hollander, Larrington and Crawford, unanimously identify Harbard as Odin in disguise in their introductions to the poem Hárbarðsljóð, the only one in which Harbard appears as a character.
The situation with Saga is a bit less clear, given the scarcity of references to this goddess. The only source of information is again Grímnismál (stanza 7, Carolyne Larrington translation), where she appears in a list of godly residences:
Sokkvabekk a fourth is called and cool waves
resound over it;
there Odin and Saga drink every day,
joyful, from golden cups
Saga is briefly mentioned again together with her dwelling Sokkvabekk in Gylfaginning (chapter 35), which probably uses Grímnismál as a source.
The fact that she is described as drinking with Odin, combined with the etymology of Sokkvabekk (sunken-bank) which is reminiscent of Frigg's dwelling in Fensalir (fen-halls), has led scholars to identify the two goddesses. The most commonly accepted etymology for Saga, 'seeress', a role attributed to Frigg seems to confirm the identification.
However, while one can definitely talk of Harbard as 'Odin in disguise', as evidenced by the fact that in Hárbarðsljóð he never reveals his true identity to Thor, it is more appropriate to refer to Saga as a by-name of Frigg (rather than 'Frigg in disguise'), possibily used by skalds for the purpose of alliteration, on which Norse poetry relied a lot. On this point, Stephan Grundy makes a little comment in Freyja and Frigg (appearing in The Concept of the Goddess):
[...] but Sága, who drinks with Óðinn in Sökkvabekkr, may be a by-form of Frigg used for alliterative purposes (though Frigg’s own name is used to alliterate with that of her hall, Fensalir, in Voluspá). Hollander suggests that the names of the two halls, Sunken-benches and Fen-Halls, may be compared, and the verse itself suggests a close permanent relationship between Sága and Óðinn: ‘Sökkvabekkr hight the fourth… where Óðinn and Sága drink through all days, glad, out of golden cups’ (Grímnismál verse 7).