Formatting part of a header property

[Br. Samuel Springuel]

I'm making use of a custom header (with custom fields) and have come 
across a situation in which I need to apply custom formatting (italics) 
to a piece of the field (just a handful of words in the middle).  Given 
that the header fields are normally simple strings, is there a way to 
format part of the string without resorting to splitting the field up 
into different parts (which could then be independently formatted)?

MWE to play with:

\version "2.19.82"


\book {
  \paper {
    indent = 0\mm
    scoreTitleMarkup = \markup {
      \fill-line {
        \null
        \fontsize #4 \bold \fromproperty #'header:custom
      }
    }
  }
  \header { tagline = ##f }
  \score {
    { s1 }
    \header {
      custom = "PRAELUDIUM I italicized text Back to normal"
    }
  }
}

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Br. Samuel, OSB
(R. Padraic Springuel)
St. Anselm’s Abbey
4501 South Dakota Ave, NE
Washington, DC, 20017
202-269-2300
(c) 202-853-7036

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