You might be able top find an older Brno 602 in .458, and this like the CZ 550 is a good rifle to convert due to the long magazine.
The South African's are pretty good at improvising, and in the June 2007 Magnum Magazine there is an article describing how to get .458 Lott velocity from a Lott chambered .458 602 rifle (I can only assume it was a 602 as they refer to the rifle as a Brno rather than a CZ, but no model number was mentioned) when Lott cases or fire formed .375 H&H cases are not available.
They seated the bullets long to match the COAL of the Lott loads, and smeared a single drop of Lock-Tite 243 around the shank of the bullet to make up for the lack of a cannelure. The assembled rounds were allowed 3 hours of drying time before being fired. Loads were made up with Somchem S-321 propellant in Winchester brass. Primers were described as Magnum, but no make was identified.
Results were that the 350 gr Speer averaged 2478, 450 gr Barnes X bullets averaged 2220 and 475 gr lathe turned solids (scaled weight was 469 grs) averaged 2180.
Naturally there were the cautions at the end of the article that said that these procedures should only be attempted by very experienced handloaders and that the use of nonstandard loading techniques could result in a catastrophic failure.