Person names are broken down into parts so they shouldn't be formatted. One way around this is to indicate the presenter with additional bibliographic data. Here we put the presenter's rank in the author list in the usera field. The first author is assumed the presenter when no rank is given.
\documentclass{article}
\usepackage[style=alphabetic]{biblatex}
\usepackage{xpatch}
\newbibmacro*{name:emph}{%
\ifboolexpr{ test {\ifcurrentname{author}} and test {\ifbibliography}
and test {\ifentrytype{inproceedings}}
and ( ( test {\iffieldundef{usera}}
and test {\ifnumequal{\value{listcount}}{1}} )
or test {\ifnumequal{\thefield{usera}+0}{\value{listcount}}} ) }
{\itshape}{}}
\xpretobibmacro{name:last}{\begingroup\usebibmacro{name:emph}}{}{}
\xpretobibmacro{name:first-last}{\begingroup\usebibmacro{name:emph}}{}{}
\xpretobibmacro{name:last-first}{\begingroup\usebibmacro{name:emph}}{}{}
\xpretobibmacro{name:delim}{\begingroup\normalfont}{}{}
\xapptobibmacro{name:last}{\endgroup}{}{}
\xapptobibmacro{name:first-last}{\endgroup}{}{}
\xapptobibmacro{name:last-first}{\endgroup}{}{}
\xapptobibmacro{name:delim}{\endgroup}{}{}
\begin{filecontents}{\jobname.bib}
@InProceedings{bart,
author = {Bart-Smith, H. and Bastawros, A.-F. and Mumm, D. R. and
Evans, A. G. and Sypeck, D. J. and Wadley, H. N. G.},
usera = {2},
title = {Compressive Deformation...},
booktitle = {Porous and Cellular Materials for Structural Applications},
pages = {71--81},
editor = {Schwartz, D. S. and Shih, D. S. and Evans, A. G.
and Wadley, H. N. G.},
volume = 521,
series = {Symposium Proceedings},
location = {Warrendale, Pennsylvania},
year = {1998},
publisher = {Materials Research Society}}
@InProceedings{arzt,
author = {Arzt, M. and Brocks, W. and Mohr, R.},
usera = {3},
title = {An implicit integration method...},
booktitle = {Proceedings of CMEM 99},
pages = {371--380},
year = {1999},
editor = {Carlomagno, C. M. and Brebbia, C. A.},
volume = {2},
series = {Computational Engineering},
address = {Southhampton},
publisher = {WIT Press},
isbn = {1-85312-683-7}}
\end{filecontents}
\addbibresource{\jobname.bib}
\addbibresource{biblatex-examples.bib}
% just for demonstration
\ExecuteBibliographyOptions{firstinits,maxbibnames=99}
\DeclareNameAlias{default}{last-first/first-last}
\begin{document}
Filler text \cite{moraux,companion,bart,arzt}.
\citeauthor{moraux,bart,arzt} showed that...
\printbibliography
\end{document}
