Can o-phenanthroline be used as a complexing agent for lead(II)?
Yes, there are numerous known lead(II) o-phenanthroline complexes with thiocyanate, perchlorate and bulkier organic ligands.
As for nitrate, Bytheway et al. [1] reported synthesis (from lead(II) nitrate in methanol) and crystal structure of bis((1,10-phenanthroline)-(nitrato-O,O'))-lead(II):


Figure 1. crystal structure of $\ce{[(phen)2Pb(NO3)2]}$ (CCDC: 1246661, CSD: RAQMAX).

Unfortunately, there is no UV-Vis/NIR spectra available.
Harrowfield et al. [2, p. 1030] reported that the crystals are colorless, which likely eliminates visible region of detection.

References


Bytheway, I.; Engelhardt, L.; Harrowfield, J.; Kepert, D.; Miyamae, H.; Patrick, J.; Skelton, B.; Soudi, A.; White, A. Lewis-Base Adducts of Lead(II) Compounds. XIII. Synthetic, Structural and Theoretical Studies of Some 2:1 Adducts of 1,10-Phenanthroline With Lead(II) Oxoanion Salts. Aust. J. Chem. 1996, 49 (10), 1099. DOI: 10.1071/CH9961099.
Harrowfield, J.; Miyamae, H.; Skelton, B.; Soudi, A.; White, A. Lewis-Base Adducts of Lead(II) Compounds. VII. Synthetic and Structural Studies of Some 1:1 Adducts of Linear Polyamines With Lead(II) Nitrate. Aust. J. Chem. 1996, 49 (10), 1029. DOI: 10.1071/CH9961029.