  For a sub-population of energetic Gamma-Ray Bursts (GRBs), a moderate
baryonic loading may suffice to power Ultra-High-Energy Cosmic Rays (UHECRs).
Motivated by this, we study the radiative signatures of cosmic-ray protons in
the prompt phase of energetic GRBs. Our framework is the internal shock model
with multi-collision descriptions of the relativistic ejecta (with different
emission regions along the jet), plus time-dependent calculations of photon and
neutrino spectra. Our GRB prototypes are motivated by {\em Fermi}-LAT detected
GRBs (including GRB~221009A) for which further, owing to the large energy flux,
neutrino non-observation of single events may pose a strong limit on the
baryonic loading. We study the feedback of protons on electromagnetic spectra
in synchrotron- and inverse Compton-dominated scenarios to identify the
multi-wavelength signatures, to constrain the maximally allowed baryonic
loading, and to point out the differences between hadronic and inverse Compton
signatures. We find that hadronic signatures appear as correlated flux
increases in the optical-UV to soft X-ray and GeV to TeV gamma-ray ranges in
the synchrotron scenarios, whereas they are difficult to identify in inverse
Compton-dominated scenarios. We demonstrate that baryonic loadings around 10,
which satisfy the UHECR energetic requirements, do not distort the predicted
photon spectra in the {\em Fermi}-GBM range and are consistent with constraints
from neutrino data if the collision radii are large enough (i.e., the time
variability is not too short). It therefore seems plausible that under the
condition of large dissipation radii a population of energetic GRBs can be the
origin of the UHECRs.
